If you're frequently distro-hopping, I recommend using a seperate /home partition. I did that before I settled down, I can't begin to describe how convenient it was (especially if you use Flatpak).

Copy them to an external drive or another computer, copy them back after.

Chances are you're gonna wanna wipe the partition table on your switch over so I'd just copy them out then back in. No point over complicating things.

I'd recommend starting fresh. Make a new one but don't delete the old one. You can then copy over what you want without bringing over anything like dotfiles with bad settings.

If /home is on a different partition just don't format it and set it to mount on the same place and you should be good to go. If it's not make a backup, then create a partition just for it, install your new system, restore the backup, and next time you won't need a backup.

WHY THERE ARE NOT.

Please have a partition for /home. In fact, you need partition for /usr, /var,.. too

Just remove every directory except for /home/. Then install the new OS without repartitioning.