There was discussion about this when Infinity War came out last year, but they should have kept Thanos’ original motivation for wiping out the universe from the comics; to please the goddess of death. A child who suffered abuse on Titan from disfigurement who would continue the cycle on his own “children” and the universe to receive what he believes to be some semblance of acceptance.
MCU Thanos is such a weird character who’s primary character development is his relationship with his “daughters” and that’s about it. Thanos was the protagonist of IW, and has a defined arc within it, yet the “universe is overpopulated” bit doesn’t feel convincing. I’ve seen some complaints here and elsewhere about him in Endgame, but he works so much better as an obstacle than as a real character.
“I am inevitable v. I am Iron Man” works in the moment, but it could have been a much more powerful refutation.