Board Game Arena

I think "rules change" in Red7 is a bit of a misnomer. It's really just one condition that changes (compared to the Fluxx family of games, for example). It's a neat little puzzle, but it also can feel like playing solitaire sometimes.

I see Carcassonne has been mentioned, some other tile placement games are Kingdomino, Cacao and Sapiens.

For 2p in particular, I'd suggest Targi.

Chakra hasn't been mentioned yet and absolutely fits the requirement of 'quick to learn; hard to master'.

Butterfly is a great game that's simple to learn and play but with lots of strategic depth.

I don't know if you'd call it abstract or not - you're a Butterfly collector collecting tiles for points