It will really depend on use case. The most straight forward one is using LineageOS and F-Droid. With more specifics, I can give you a better fitting setup.
Yeah IDK if you are even able to get the bootloader onlocked for your version. Regardless, there is still a lot that you can do with stock. You can disable most of the Google stuff through ADB and install F-Droid. Neo store even has backgroud updates for Andrpid 12+ IIRC. Here is a short guide: https://www.reddit.com/r/degoogle/comments/jqxe1u/tutorial_how_to_degoogle_any_android_phone/
I can't vouch for performance or update breakages though.