After installing your OS (Manjaro in my case) and configuring it to your likings, which directories do you backup? (or reverse-question, which directories do you not backup?)

Assuming you're using btrfs, just use snapshots. You can also make use of Snapper to automate the whole thing, including setting up hooks for pacman so that it automatically takes a snapshot before an update.

And for portability/reproducibility of your OS config, just backup your dotfiles:

rsync -avh --exclude={'.local/share/Trash','.Trash-1000','.cache','/[^.]*'} ~/ /mnt/backupfolder/ --delete

and package list.

And for your personal documents, just use your favorite own/cloud service provider's tool.

/etc and ~/.config in priority

I think that you don't need to backup /var or /tmp

.mozilla

One of the reasons why I will probably never install discord/slack/element for desktop, and I will use the browser version is because it's nice to have everything in one, easy to move around place.

Since I have multiple profiles I can't really take advantage of the firefox sync service.

~/Documents

Notes, poetry, other work.

~/.ssh

So I can access my servers from any machine.

~/Playables

Games and related data. Although this one is of significantly less importance, since I mainly play only ome game, binding of isaac, a roguelite.

Exclude: /dev, /proc, /sys, /tmp, /run, /mnt, /media, /lost+found, /swapfile (not a dir), ~/.cache, ~/.local/share/Trash Include: ~/.cache/paru

When using a pre-configured Arch derivative, I really only bother to back up a few folders in /home, e.g. ~/.config, ~/Downloads, ~/Documents, ~/Pictures, ~/Music, maybe also .bashrc if I added a bunch of aliases. Everything else is easy enough to reinstall.