Thing with Upamecano is he's having immense potential, but as a fan you never feel calm with him around. You just know he can fuck up at anytime.
I still haven't given up on him, but he really needs to stop these moronic mistakes come next season. He can play like a GOAT CB at one match then the next he's like the worst turd. I can't say if Upamecano is better than Bremer, I don't watch Juve matches on a regular basis.
Pavard is nowhere near having the same potential, but he's a much more stable player - not to say he doesn't make mistakes, he certainly do. But he's way more consistent in his performances.
Hernandez should be sold, injured all the time and we can get better in that position anyway.