I mean, I just plug the drive in once every week or so, move any new personal, irreplaceable files to the drive via whichever file manager I fancy at the time, and then set it aside for next backup.

There's no replacement for physically backing up your data. Automation can even be the cause of file loss. Take it from someone who has spent days recovering their files via disk recovery tools.

External drives are camping kits for PCs. If you have one, then it doesn't matter if you lose your system, just reinstall or install something new, open your camping kit and make camp. Make a dotfiles repository if you want to save your home and app configs.

Windows and Mac is like a long term home ownership with a car, kids, partner, and too many bills to be free again. Linux is a nomad life. Nothing is for certain and you could lose your tent in a thunderstorm if you don't stake it down properly.

Also, Timeshift is a very rudimentary and first-layer protection. Something that got configured wrong could have been configured wrong months ago and you may not have caught it at the time and all the restore points you've kept could have the same problem.