Best Way To Buy Games For PC & Xbox

My son plays all games on his gaming PC. I will be playing all the games on my Xbox where my son also has an account but never plays. It's our home console. I'm trying to avoid having to buy a game twice, one for PC and one for Xbox. I know there's the "Play Anywhere" that some games support but I'm noticing most of the games we are interested in are not on that list.

Is there a way to purchase the game so that we can play it both on the gaming PC for my son and on the xbox for either one of us to play? We also subscribe to Game Pass Ultimate but that's a different animal altogether. Just wondering if there's a method or process to ensure we can play on both devices.