IME the battery isnāt the issue, especially if you get a phone with a big battery to begin with.
Itās the lack of updates thatās the issue which means you can no longer use certain critical apps (banking, government, etc.) on your phone.
The phone becomes obsolete before the battery ever dies.
Know any banking apps that work with a custom rom?
What about your national governments designing an identification or tax related app that works on a custom rom?
Good luck with that.
They donāt have to. Nobody designs their apps to work with specific ROMs, they design it to work with Android. Custom ROMs are still android ROMs, theyāre just managed by open source communities instead of phone OEMs.