Bohm isn’t getting Brown on his own, let alone having to take Castellanos, let alone adding another prospect. Come on! I think you are grossly overvaluing Bohm. He has one year left on his contract too.
Think about this from the Cubs POV. Do they want Bohm and Castellanos plus their salaries and losing Brown and a prospect, or would they be better off with something like Andujar and Hays as free agents for less salary and not lose any players. I wouldn’t even do Bohm for Brown straight up. Again, rather have Andujar and keep Brown. If Bohm is cheap, fine. If not, hard pass.