Doing that becomes harder and harder as time goes by. When I was shopping for a new Android phone I couldn’t find one that was recent enough and also had custom rom support of any kind. I know there are smartphones specifically built to be friendly towards custom roms and Linux but those are only available in Europe and North America.
Honestly, when I first heard the term “boomer shooter” I thought the games were called that because of the amount of explosions they have. Ion Fury in particular is full of them. It a was a while after I realised that by “boomer” they basically mean “old”.
I think “retro shooter” is the much better term, though “classic” also works.
I’m sure hardware is fine, I had a budget phone from six years ago (Huawei Y7 2019) before I upgraded and while it wasn’t good, it wasn’t unusable either. And that phone was just a hair above an Android Go phone. At this point performance upgrades should slow down. I can’t speak for their software support though.