For backstory: My partner and I have been playing Super Mario Party so long that it’s gotten boring. They asked me what I wanted for my birthday (which is coming up), and I thought something like that would be cool. If X-COM 2 had a local co-op on the Switch, I’d do that, but is there something similar?
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.