I just do the Micorsoft rewards with Bing searches and gamepass quests and I haven't actually ever paid for GPU. I just rack up enough points to get a 3 month pass, and redeem it. I am good to go until next January. It takes me usually 1 1/2 - 2 months to get the points needed so each time I am slowly pushing out the re-up time by an extra month.
I currently have around 20K rewards points. I'm saving those up for when I get an Xbox. Probably in the fall. By then, I should be good to go for some time in the Game Pass.
Don't you get rewards points by getting achievements as well?