I just don't subscribe to the idea that Moncada's defense is so bad that he needs to be moved. His range stats are top 3 in baseball for second basemen and I trust that he'll iron out the misplays on easy balls.
I don't see Anderson ever getting to .800, to answer your question, but I do see him getting to .775 or so, which, in addition to his newfound defensive prowess and baserunning value, does make him valuable overall. However if anyone is getting moved or traded for Madrigal, I would guess that it's Anderson and not Moncada.
Anyway, I agree that OPS is not the best stat to judge a player like Madrigal, but I still question that he'll be a more valuable 2B than Moncada long-term, especially if he doesn't walk.