I liked Half-Life, Sins of a Solar Empire. Hit me.
I had my work macbook pro with me during a vacation in 2013. Enjoyed playing Darwinia, Portal 2, and Tank Universal when not at the beach. All 3 are absolutely fantastic games that do a bit of genre bending. You’ll want a stand alone mouse for them, trackpad integration is a bit funky.
Is dual booting Windows an option for you? 'cause then if so you’re set for life. I’m not sure if these games all have Mac Os counterparts, but many do:
You liked sins, so i’m going to suggest Star Craft 2 if you haven’t played it and Dawn of war III, Home world remastered, coupled with the new prequel: Deserts of Kharack, for that RTS itch, and Stellaris for the 4X with a bit of grand strategy thrown in. There’s also Civ, of course.
Rimworld is probably the top sandbox game these days. People seem crazy about it if they are into that kind of thing.