assuming backup the home folder also
If you've put anything significant in there, yeah, it's not a bad idea to backup your stuff. Timeshift doesn't usually backup the home folder; it usually takes daily/weekly/monthly snapshots of system folders.
I’ll be using it as a general browser / social/Lemmy machine but also for just learning Linux, structure, CLI and getting familiar with distros/DEs. Maybe a bit of dabbling in development.
Mint should do pretty well for your use case, at least as far as learning the Debian side of the family goes. The main drawback to Mint is that it's not bleeding edge -- which is also sometimes a strength. For old hardware, it's generally pretty good; for new hardware, I've had issues because of things like the kernel being too old and not having the drivers I needed. There are ways around that though like using the OEM/HWE kernel releases -- which is what I ended up doing when I put a modern GPU into my ~10 year old desktop.