the most important thing is your user files. everything else just speeds up recovery.

you should keep a bootable recovery medium around, like an installer USB, so you don't have to bother your neighbours for one at 2 in the morning..

to restore faster, you either make disk images (can restore everything quickly in one go) or save partiton layouts, configs and package selections as well as everything you installed without package management. if you don't do this second part, you have to sit through a reinstall and figure out everything again and that sucks if you don't have time. like you really need to open that document, but you forgot the name of the program you use to edit it etc..

if you use just one large file system, you can tar everything up, using --one-file-system , so it skips stuff like the inside of mounted snap packages, which also are present in another place. on restore you then have to format untar and install a boot loader. beware that EFI boot can be difficult to set up and has another partition. so this is just for pros. however, this enables to use tar features like differential backups.