Trainer mode/room for major bosses

Probably suggested before, but I didn't find a thread.

I'd be great to have a trainer mode/room for major bosses. Where you don't earn/gain anything from the encounter, but you can repeat the encounter as many times as you want.

My main reason to not engage with major bosses is the 6 portals and you are out. Which is fine for the encounter itself, but not for learning the encounter and checking how the build you are running can deal with the encounter.

I think delve bosses did that much better (although I think you are not allowed to die?), since you can repeat the encounter until you finish it at only the cost of sulphite. But anyway.

Ideally have a room where you can spawn a major boss encounter as many times as you want (keep the 6 portal limit), no loot, no exp gain, no quest progression, no achievements. Add bosses that you have attempted to the room, if you are really stingy only add bosses that you have beaten already (would still be better than nothing). If you are really, really, really stingy, add some currency (somewhat common) that lets you initiate the training encounter.

If that is too much work, just add some new mode for the atlas interaction, that disables all the mentioned things but doesn't consume your fragments/items that are required to initiate the fight. If you are really, really, make it cost chaos orbs.

By "major bosses" I mean the ones from the atlas story. Side content from leagues or from the act 1 and 2 quests you don't need.

Personally I think it'd still be better (on average) to sell the encounters even with a trainer and guaranteed kill in the "real" encounter (even without dying). But at least you'd be going in prepared and any death will be on you. Not due to unga bunga mechanics you must read on the wiki / watch a video for.

This is coming from my "enjoyment" of more and more "final" bosses that get added to the game. I think the encounters are fine, some are even enjoyable. But the 6 portal limit really hinders learning the encounter and usually leads to me avoiding the encounter until "later on" (when I get really bored with the game).