Fine, slot him at 3B - but dont trade Casas IMO. Well, it depends on return but if you trade Casas you are cutting into your flexibility.
If Yoshida becomes a massively overpaid bench player, its a sunk cost. Most likely, there will be some kind of injury or bust somewhere , and we'll be well suited to juggle things around and maintain success if we have Bregman, Casas, Devers, Yoshida, Story, Campbell....
Devers to 1B and Casas to DH can be plan A, no issues with that. But if Story gets hurt or Campbell needs more time, or maybe Campbell moves to OF if RA needs more time....
My only point is that I value Bregmans flexibility and would welcome his signing even if we dont immediately trade Casas. Because trading Casas cant be undone. Where you slot Bregman on the diamond can be undone.