You can use OnFailure
in your .service
file if you want some thing to happen when that specific service fails, but I don’t know if there’s a blanket way to tell systemd to notify via email when any failure happens (I wouldn’t mind a desktop notification… will investigate)
(I assume you meant “I created a separated /var partition”)
You can move/resize partitions from basically any live usb (via cli or gparted for gnome and kde partition manager for kde).
Shall you want to, you can also merge the var partition with (say) your root partition:
Be aware that you can very easily lose your data ;)
PS: just in case, try running
flatpak uninstall --unused