I'd argue Thuney is the only one who is a must-sign whether Brady is back or not...there's a point at which it's "too much" of course, but I would absolutely pay him something like the Brandon Brooks contract (4/$56/$30gtd). It can be structured to minimize the cap hits in years 1 and 2, which is important if you are paying Brady. If you are not paying Brady, then the cap math matters less, but he is an important piece of the line and he's basically never missed a snap. It looks like they have a legit LT on a rookie deal for a few years yet.
If Brady IS back, then I think trying to fit in some of those vets (KVN or Collins, McCourty, ST boys, Shelton/Butler) is important to making a real run, and doable if they take some abuse on the cap for 2022/23. No matter how much they "hit" on the draft, they aren't replacing players at every level of the defense, AND the OL, AND skill positions, AND special teams enough to be truly competitive. That's an insane position to take. There is some restructure flexibility with a couple guys, some guys who are likely cut (Ghost? Sanu?) because the money can be used more efficiently.
If Brady is NOT back, then you keep Thuney, and that's it. Maybe some of the down-roster OL guys or Butler (shouldn't be too expensive) also just because you have to field a team, but if they are rolling with Stidham they should be honest that he is not going anywhere special with the same roster Brady just had, but a year older.