Sinner has not necessarily 'maxed out' though he has reached a dominant, elite level that makes further improvement difficult
Sinner has demonstrated immense maturity, improved his serve significantly ( increasing ace count by 50% from 2022 to 2024) and shown top tier resilience in high pressure moments, I feel he is more consistent than Alcaraz in ways.
I also felt he used good tactics in defeating Alcaraz recently in MC, pinning him in his bhand wing, not being able use his fhand, at one count Alcaraz had 14 UFE on his bhand, in the match
I feel he can improve his physical endurance for longer matches, his 5 set record (6-10), lower than his rival Alcaraz
He has indicated the ability to continue to evolve his game rather than having reached a permanent ceiling