Which are those? Does he become faster, less rash and more reliant in possession once he has a more reliable partner? Our midfield set up does showcase him in the worst possible light, highlighting all of his weaknesses, but those don't simply disappear with a more competent partner next to him. You can, of course, hide him behind a couple of great midfielders Pirlo-style, the issue is, does he have enough creative output to justify it?
And yes, he still is, at least as the things currently stand, out best central midfielder (not sure why Mainoo forgot how to play there but he did, Bruno was never a central midfielder in the first place and Ugarte has turned into a pumpkin).
With a competent partner he can focus solely on his own game and compensate for his own weaknesses, weaknesses I'm sure he's acutely aware he now has (we're all very aware of our own decline). A lot of the time, he is overextending himself here and trying to cover too many poor aspects in his partner, which leads to some of the rashness and more bone-headed plays. The thing is, most of the time, those supposed bone-headed actions are to cover for someone else not doing their job or simply not having the prescience or quickness of mind to process what's about to happen due to their lack of positional awareness or understanding, so Casemiro will take the card for them or do what needs to be done in lieu of the partner's failure to react accordingly. Imagine the vast majority of that coddling being washed away and him only having to focus on his own game. It would be transformative for him and the team.
His legs are in the process of going as is his stamina, but he still has the innate understanding of midfield to conserve his tank if left to his own devices, so yes, less rash for certain given the majority of the time he is having to be rash is the fault of whichever player is out there next to him. A better partner also gives him more time on the ball by better sharing the load and drawing men to him or simply being available as either an outball or someone to interact with, and given a competent partner will by default be majority in the correct positions to be viable, it would result in Casemiro himself being able to better select what he himself will do, or even have more time to look up and pick out a better option, so yes, more reliant in possession too.
One higher mind in CM can not ward off two or three good minds by themselves. Two higher minds can, though. Or even a higher mind next to a good mind stands a chance. We have a higher mind next to: Bruno, who isn't a CM and doesn't have understanding of the nuances of the role, who often leaves his partner all out at sea; Ugarte, who is simply not good enough to play in a 2-man midfield in the PL; Mainoo, who is deficient athletically and likely to struggle with coverage sooner than later in one half of football, let alone two. In each instance, Casemiro is by himself and having to cover for others leaving him in the lurch in their own particular way - his energy and game should all be front-facing and outward toward the opposition and the game going on around him, not to, by default, having to carry CM partners who are seriously deficient in one way or another that thus have him focusing as much on them and what they're not providing, as the opposition, and what they are.
Casemiro is miles off what he used to be, but still, it would have been nice to see what would have happened here if he wasn't continually carrying midfield partners and was allowed to focus on himself and the opposition in its entirety. I think we'd have seen that whilst he was no longer modern legend Casemiro, he was still way above most others during his slide.