What's the best way to backup a phone without root?

I've got a Mi 10t Lite 5g that's starting to show its age, and could do with a factory reset to clear some of the crap. Ideally I want to unlock the bootloader and try some custom ROMs too.

It's been years since I did a backup without moving to a new phone, so I don't know the best way to do it anymore. What's the best way these days?