Perhaps good to know is that the Pixel tablet will get about two and a half years of security updates from google . Since GrapheneOS relies on these updates, you will get the same time of their updates. After that, they offer extended (but limited) support.

I’m not sure if it’s a grand strategy game, but Freeciv is similar to older Civ games.

That being said, Beyond All Reason (BAR) and Zero-k are Total Annihilation inspired and have a lot of work put in them. Definitely worth a try!