Unleash Your Demons

Unleash Your Demons by Keira Marcos is an amazing Avengers story that goes AU after Infinity War (and it's a much better resolutions than Endgame... *shudder*).

Here is the author's summary: Tony Stark can't live with the damage Thanos has done.
Infinity Stones in hand, he makes a choice that will alter the fate of
the universe.