I think I've just been irreversibly damaged by Path of Exile's fast gameplay, D4's combat felt pretty tedious to me in comparison. Or maybe I've just had the wrong builds so far. I've been meaning to give it another shot but there have been so many other good games recently so I guess it'll have to wait.
I know there’s a mentality out there that a Diablo game is a failure unless it’s the only game you ever want to play for the rest of your life, (which, btw, I don’t even feel that about Diablo 2), but it’s ok to take breaks and play different games and come back to a game when it’s updated.
Yeah I don't get that either. I have to skip leagues (seasons) in Path of Exile both because I wouldn't have enough time to play other games if I didn't and also because I tend to play it so much that I get tired of it and need a break afterwards.
It sounds like you're actually looking for turn-based tactics/strategy? In that case maybe check out Civilization VI and the Advance Wars 1+2 remake. They're both supposed to have local multiplayer on the Switch, although I don't know how good it is. I don't think you can play the Advance Wars campaign co-op, but it sounds like you can at least play individual battles like that.
If you're really looking for real-time strategy, Pikmin 3 seems to have a good co-op story mode. Pikmin 4 technically has one too but the second player isn't able to do much, so it's probably not very fun.