Rewritten Synthete is way op 3.8

I'll start off with the parts which can be fixed very easily and move over to those which come after a while.

Flasks:
-You want a divine life-flask for emergency purposes. Your minions usually kill enemies fast enough to sustain your charges anyway, the instant-flask provides a chance to survive any follow-up attacks which you'll succumb to if you manage to be alive after the first shot. To increase the survivability against one-shots I'll add something in a bit, don't worry.

-All your utility flasks should've 'reduced charges used'. Not only does it allow you to sustain 100% upkeep of flasks, it also makes every single charge worth more, hence each mob will provide you with more sustain then you would have otherwise.

-Get your flasks to 20% asap, this is one of the most important things to do!

-Quartz Flask: Bleed immunity. Why on there? Because the life-flask just removes it in a moment, the quartz-flask will be up indefinitely anyways and provide a hefty boost of defense.

-Quicksilver Flask: Free immunity, it has the most amount of charges of all your utility flasks present, hence this should be on there. Also it allows you to get rid of Dream Fragments for an actual useful item.

-Granite Flask: Switch it out to instead get a Eternal Divine Mana Flask, as to why I'll explain it in the next section, the 'sustain' one which will focus on survival for you and your Minions.

General Sustain:

First off, offense versus defense. This part is extremely important. Since PoE-builds usually handle encounters in the manner of 'the longer a character needs to clear an encounter the stronger the defensive measures need to be' you have to think what a summoner does.
Minions rush out when the - still a bit clunky - AI starts to recognize a threat. This only happens for roughly the distance of your visible screen and lets them re-target afterwards if more enemies are nearby. They won't actively attack enemies outside your screen if they haven't engaged an enemy before and killed it already.

So this means you'll need quite a bit of ability to survive engaging enemies as you're usually going fairly close to them at least once.

To make this happen:

Take Mind over Matter on the tree, as a summoner you're barely self-casting outside of your skeletons and offering. This makes you able to keep a large portion of your mana to bolster against any incoming damage. This is also the reason for the mana-flask, permanent sustain which wasn't possible pre 3.8.

You need a defensive skill, preferably even 2. At the moment there is neither a self-cast nor an automatic defensive measure on your build, this makes your character very very squishy. Since the Necromancer-Ascendancy doesn't provide innate defenses which are beyond the 'base' level you need to have some other measures to take care of that.
The easiest ones to do this are:
-Stone and Flesh (Since you're usually not in melee-range, and even if, the chance for enemies to hit is low)
-Immortal Call. No, not Steelskin, I'll explain why:
Steelskin is a great skill to mitigate incoming damage in a short amount of time, a second blast after a close one-shot. This is nice definitely. Though since the amount of damage mitigated usually is beyond what the skill can handle the uptime is fairly low. Hence the 'better' version is to mitigate incoming damage to a degree over a longer period of time --> This is done via Immortal call, you're highly defensive against elemental and Physical damage during the uptime.

Also, last but definitely not least: Focus your build in life, don't bother with ES unless you've got very good gear and can handle to invest into a proper Hybrid-build. ES is only useful during short fights with the low amounts a Summoner usually is able to have, since ES-regen is based on the total amount of ES you have it only becomes a meaningful thing with heavy investment. We summoners usually don't have that.
Which brings us to out next topic:

General build issues:

Your build has a theme I can take out of it, sadly it doesn't follow it properly.
Seeing your setup and general direction I can see you're going with Solar Guardians as Spectres, so you're playing a Spectre-build. Nonetheless you're going with Minion Instability and Skeletons as a secondary means of damage. This is a negative, it's the focus of Popcorn-builds. You don't want that in your setup. Also your general build lacks a few important parts and takes a few non-necessary ones spent better otherwise for more effect, I'm going into that specifically in the passive-tree section.

You're not focusing on either ES or Life. Life is the easier method to survive while ES is the superior one with heavy investment. I recommend ignoring all ES-nodes completely and focusing on taking the Life-nodes you've ignored on the tree. That'll help out a lot.

Your equipment isn't focused on the gem-setups you have. I'll go further into it in the equipment-section if my suggestions, you'll see what I mean with it then. At the current state your build ignores all the major important increases which are in your reach.

Decide properly if you want to do a Popcorn build (SRS + Skeletons) a fire-based build (Spectres + SRS) or a physical build (Skeletons + Zombies + auxiliary Spectres). At the moment you got a combination of all of those which turns out to be fairly mediocre at best.

Gem-Setup:

I'll give you several options of viable setups to focus your build in a single direction instead of multiple ones:

Fire-Summoner (Solar Guardians):

6L Spectre: Summon Spectre/Minion Damage/Elemental Army/Feeding Frenzy/Minion Speed/Deathmark (For focusing on sturdy enemies)
6L SRS: Summon Raging Spirits/Minion Damage/Elemental Army/Feeding Frenzy/Minion Speed/Added Fire Damage
4L Movement/Auxiliary: Flame Dash/Faster Casting/Desecrate/Flesh offering
4L Golem: Flame Golem/Minion Life/Meatshield/Minion Damage
4L Survivability: cwdt/Spirit Offering/enfeeble - non-connected Anger
1L Unset-Ring: Stone and Flesh

This setup focuses on dishing out as much fire damage as possible while letting you survive when you get hit by giving you a sudden chunk of ES+reducing the incoming damage of close-by enemies. All minions are fire-based and thus deal the most possible amount of damage available while your golem and SRS is supposed to clear enemies making it close to you, your Spectres will clear out all enemies at a range very reliably.

Physical build (Skellie or Zombie focus):

6L Zombie or Skeleton (I recommend Skeletons hence I'll write it like them): Summon Vaal Skeletons/Minion Damage/Brutality/Feeding Frenzy/Melee Physical Damage support/Minion Speed
3L Spectre: Summon Spectre/Meat Shield/Minion Life
4L Zombies: Summon Zombies/Meat Shield/Minion Life7Immortal Call (Self-cast)
3L Survival: cwdt/Spirit Offering/Enfeeble
4L Movement/Auxiliary: Flame Dash/Faster Casting/Desecrate/Flesh offering
4L Golem: Summon Carrion Golem/Feeding Frenzy/Maim/Brutality
1L Unset-Ring: Stone and Flesh.

This setup is pure physical, your Minions are supporting each other and you'll have Host Chieftains/Carnage Chieftain as auxiliary support for charges. Your skeletons can be target-cast hence you won't need to have Deathmark on your minions. Zombies barely every die and Golems support the others with maim to increase the overall damage-ouput. Beware though, your golems are slightly squishy. If you want to take care of that switch Brutality with Minion Life.

Passive Tree:

First of all the general things:
You want your minions to do as much damage as possible while surviving as long as possible while also yourself surviving (obviously since they won't do damage otherwise).

To achieve this the following changes are far superior to your setup at the moment, I'll speak from a life-based setup since MoM is stronger then non-focused Life/ES builds at the moment.

So this is how it goes to increase the overall sturdiness as well as the damage and life of your minions at the same time:

Remove Whispers of Doom (+3 points)
Remove Minion Instability (+1 point)
Remove Unnatural Calm (+3 points)
Remove the extra point at Death Attunement (+1 point)
Connect between 'Enduring Bond' and 'Heart and Soul' for 2 points used, instead remove the whole left side after 'Deep Wisdom' to gain 17 points (+15 points)
Connect 'Purity of Flesh' and 'Retribution' over the minion-nodes. (-7 points)
Connect 'Redemption' on the large minion-cluster (-2 points)
Connect the top jewel-slot and 'Ravenous Horde' over 'Lord of the Dead' for 3 points, instead remove 'Arcane Focus' to gain 4 points (+1 point)
Take 'Mystic Bulwark', spell damage is your worst enemy and it provides extra mana-regen and amount to survive easier (+2 points)

After those core-changes you got a leftover of 16 points to use, those go there:

Left jewel Slot to make up for the lost one = 2 points
Quick Recovery = 3 points
Grave Pact (5% MORE damage, this is massive, also % damage and attack-speed for phys, the last is useless for solar guardians but still viable because of the other ones) = 5 points

Now you can connect of the jewel Slot to 'Constitution' and remove the other connection, this is -4 to connect and +3 to remove for a total of 1 point spent for a jewel-slot.
With the last 5 points go to 'Heart of the Warrior'.

This provides you with roughly 8,5k EHP without considering Spirit Offering or other defensive layers.

Equipment:

Weapon:

For Solar Guardians your 6L Soulwrest is ok.
If you want to go physical I recommend 2 Convoking Wands for massive boosts. You could also go with a Soulwrest 6L and get rid of Zombies entirely to summon Phantasms, in that case you'll need have a non-linked 4L to push the other gems in there.

Rings:

Left ring: Upgrade to a 10c ring at least, yours is very bad. Focus on res + life and possibly Dexterity. Craft Minion Speed on it.
Right ring: Don't use Dream Fragments, take a rare ring. Again, same stats.

Amulet:

Rare, life + res + Dexterity and best-case one like mine with a delve-mod. The last is extremely expensive, otherwise craft (if possible) a minion-craft on it again.

Gloves:

Life + res... if you've got a good chunk of money then use 4L white-socket Triad Grips instead. Obviously you'll need to get rid of 'Brutality' if you're going physical and not Solar Guardians.

Boots:

30% Movement, Life, Res.

Belt:

Stygian Vise! Get one, you're not full invested on ES, a Styg is vastly superior and very cheap.
Worst case get a few ilvl 86 bases and slam the following combination on it for fossil-craft: Pristine/Prismatic/Bound
If it rolls good res + life great! Minion life is a bonus which isn't needed but nice.

Helmet:

At least +2 minion, life, res.

As for Annointing things:
Allocate Grave Intentions. That's the strongest one you can do.
For towers: Double Meteor and Smothering 10% damage reduction (If something comes too close to the pump you need to personally survive easier as bosses can be hard)


If you follow those guidelines your build will manage T16 Blight without towers (unless some BS combination happens). Generally you'll be good to go until roughly depth 300 in delve as well.