What directories should I backup before a clean install of Ubuntu 20.04?

I am planning on upgrading my machine to the new Ubuntu 20.04 LTS (possibly Mate) by a clean install. I have a one terabyte external disk that I plan on copying my home directory (152.8 GB) to. This is in case something goes completely haywire, I'd at least have a copy of my old home folder. I have used my current setup (Ubuntu 18.04 LTS) for about four years now and may have tinkered with files in root directory here and there to provide certain files in path and all. To the best of my knowledge, I have all of those files in my home as well.

But just to be safe, are there any other directories in root I should make a copy of? Why?

In addition to this, is there any checklist I should go through before doing a clean install?

Here is how my root looks right now,

user@machine:/$ ls --all
.     cdrom  home            lib32       mnt   run   sys  vmlinuz
..    core   initrd.img      lib64       opt   sbin  tmp  vmlinuz.old
bin   dev    initrd.img.old  lost+found  proc  snap  usr
boot  etc    lib             media       root  srv   var

Solution 1:

I would recommend you to copy/backup your /home directory.

List down the names of packages installed on your system

dpkg --get-selections > package_list.txt

You can read about How to list all installed packages.

Note: Before restoring the backup you should always take care of files (~/.bashrc and other files starting with . as prefix. In my opinion I would never copy files starting with . as some of might be system dependent) user directories.