The best thing is actually to buy a used phone that was really good a couple years ago. I bought a used Oneplus 9 for $200 last year and it’s still perfectly usable and way more powerful than I’ve ever needed a phone to be. Also older phones are more likely to be supported by third party android distributions.
Just turn it off right after it shuts down before the OS starts booting again. (Or just turn it off whenever, it’s not like there’s much chance of filesystem corruption these days. Although there is a chance of registry corruption if you’re using windows and it’s updating, which is honestly worse to fix)