Endgame boss fights suggestion/multiple attempts while keeping 1 map attempt policy

I was thinking about the difference between bosses in the endgame and campaign. I enjoyed boss fights in the campaign a lot, even though I was dying regularly (bad early game build). Significantly longer fights is what made it more interesting - right now, even in T18/T17 and +4 boss difficulty on atlas tree, the fights take 15-45 seconds. For me, the sweet spot was 2-4 minutes...

I feel like the damage of the endgame bosses is alright (still can be quite deadly), but their life could use significant boost (like 2x-4x times more). Maybe they could have some balancing coefficient so the life multiplier from waystone is not as strong on bosses as on other monsters, because then it could get too extreme...

With increased life, the boss fights should take longer, and the risk of failure increases. I understand the 1 map attemp policy, however the reason why campaign boss fights felt more substantial is because often I had to fight them repeatedly as I kept dying. Those repeatedly failed fights are often also those I find the most memorable and interesting. For this reason, I though about how to keep your map 1 attempt policy, but keep this for the bosses. The answer might be in Checkpoints.

You could have a different model for a checkpoint before the boss fights. Visually, it could have lets say 3-6 flames/lights. If the player dies during the boss fight, they respawn again at this checkpoint, and one of the flames/lights goes out. This would clearly tell how many attempts are left. Boss would reset, naturally. If there are no attempts left on this checkpoint, the player dies as usual and lose the map.

Dying during boss fights could have reduced XP loss, or not - I don't think that is so important. My suggestion is only so that players can enjoy wonderfully designed boss fights, which in the endgame don't feel interesting anymore. Also it seemed to me the implementation of this might not be too difficult (even if it was for internal tests only), as you seem to have "boss fight flag" already present, as well as spawning checkpoints right before bosses.