I don't think either of them are untradeable, just means including $. But I wouldn't make the decision based on just the one season. i.e. Castellanos hasn't been good 2 of the last 4 seasons (one of which was 2020). Schwarber only 1 of them (also 2020)... because I'm not prepared to say he's going to end up being actually bad this season. Castellanos is also one year older.
Schwarber is what he is and I'd sooner keep the LH DH who can hit 35 HRs for another two seasons (or at least one of the two) and get out from under Castellanos's longer deal before he either flat-out declines or just has another bad one-off season.
But it really depends on so many other things. You probably can't get rid of both Bohm and Castellanos. But neither of them provide enough RH power. And I'm not sure you want Castellanos to be a full-time DH for that reason (but you also don't really want him in the field, even if it's LF).
Maybe they should just move Nick back to 3B 