Almost everything you care about should be in /home so back that up. Many people keep it on a separate partition or drive to make changing distros (or reinstalling the existing one) easier.

Most of your system config is in /etc so you may want to make a copy of that too.

But the proper process on Linux is not to re-install. It should not be necessary.

On top of this, part of the reasons to use containers is that you can create and destroy them at will while leaving your host tidy and stable. I use Distrobox quite a bit for this reason.