I haven't read anything about Xiaomi tightening phone modding, but one thing I remember reading often when it came to modding systems for other devices is, "if possible, never update your system", so maybe something to keep in mind.
Auster
By this logic, demos from GOG could be posted too, since both are appetizers for something else (in Prime's case, an appetizer for the paid service), and both can be claimed.
Prime is a paid subscription, so users would need to pay to be able to get the game.
If you like reading books, comics, etc, maybe Librera Reader?
Dunno how it is on Google Play, but it's available FOSS and DRM free outside of it:
https://f-droid.org/packages/com.foobnix.pro.pdf.reader/
Playing Hollow Knight. Pretty fun so far! Just a tad too hard and far more expansive than I'd expect even a metroidvania to be, so progress is slow.
Since 10 y.o. is old enough to be a cutoff for me, PS Vita, and because modding community never stopped. You can even play some Android and PC games through wrappers in it. *-*
(Translated copypasta from the Portuguese community, yeah, but since not many English speakers understand Portuguese, worth doing)
Dunno the site, but one thing I do is to go to a site known as "AppSales" and to hide whatever titles I find with MTX with Ublock Origin filters. The process is a chore, but at least, each title needs to be hidden once and only once.
And sure, this is considering that the person uses only Google Play. I, for one, prefer to seek games in stores like Humble Bundle (owned by IGN), Itchio, and so on.
Still can't block @hexbear.net instances
Maybe don't care as much? First due to the stress it causes. Second, if you pretend to be someone you're not, people may end up liking that instead, and much like with any farce, the longer it goes, the more cracks appear.
Also, something I try to do is, without worrying much for the result, to chitchat with people. If they sound/look interested, I keep going and depending on how it evolves, I might even have gotten someone who I'm going to be hanging with for years to come. If they don't sound or look interested, I slowly drop the attempt and try it with someone else down the line.
Some are pretty weird indeed XD
In a 12-paragraphs news, with the later half being shorter than the first, the first 5 paragraphs are for praising the security changes (a pretty big red flag), the 6th is for introducing the important part (while using fearmongering), and only in the 7th and 8th paragraphs that they go through the important parts, with the method itself being basically a way to scare all but the most stubborn users.
I seriously suggest that the OP (or anyone else, really) unlock the bootloader as soon as possible, and if possible, changing the OS for anything Xiaomi can't touch.