> Even Steam Play Together, ostensibly built for this purpose, wasn't low latency enough in my limited experience (this really surprised me, so maybe I'm doing it wrong).

I've had good experience with Steam Play Together, mostly playing Unrailed (a hectic game in the style of Overcooked). I definitely forget about the remote connection while playing. We were 1000 km apart, but had quite a good connection (100 Mbit, 15-20 ms ping).