Fair enough, I misunderstood then.
I always find that kind of puzzling, even if it is pretty common.
Computer devices such as smartphones can break at any time without you doing anything wrong. The hardware can simply fail you, as can the software.
How to tackle that is down to everyone’s own priorities.
Concerning hardware …
I concluded for me personally that I want to have a mobile phone around daily. So since I got my 2nd mobile phone ages ago, whenever I got a new phone I kept the respective predecessor around as a backup phone (as long as it still worked, of course, which until now it always did, call me lucky).
I use a Fairphone 3 now as my daily driver, with my Fairphone 2 as a backup.
That takes care of a hardware backup.
Concerning data …
I regularly sync my data (contacts, messages, call log, calendar, the whole Internal Storage) on the phones to my computer with MyPhoneExplorer.
That leaves out directories like Alarms and Ringtones, which are not part of the Internal Storage directory tree, so I sync them manually from time to time. I don’t change those files much over time anyway.
I regularly backup the stuff on my computer, of course.
Concerning Apps …
I don’t have 100% confidence in integrated App + data backup solutions like Titanium Backup, and additionally this would need root, which is currently not there yet on the Fairphone 3.
I don’t need myriads of Apps, so I can simply install them again if needed. From time to time I copy all their current version APKs to my computer with MyPhoneExplorer.
That makes going back to earlier App versions in case of regressions after App updates easier, and even while I would have to install them one by one again when restoring the phone, installing is faster with APKs than via the respective stores the Apps are from.
Concerning the OS …
On the Fairphone 2, I regularly backup the state of the OS with the TWRP recovery. But I could just make sure I have install files of the OS ready, too. The TWRP backup is more of a habit from the past, and it restores settings.
On the Fairphone 3, currently there’s only factory resetting the phone, until Fairphone release the announced complete image of the OS to install with the recovery. Settings are a chore, admittedly, but I could document my changes if I cared to speed up restoring.
So … In case of need, I just get the OS running again, browse through the Settings tree briefly, install all the Apps again, and then just push all the data back from the computer to the phone.
Everything practically not covered by this OS → Apps → data backup approach would be a single point of failure on the phone and seriously in need to be addressed (i.e. done differently, perhaps omitting the phone out of the use case).
Same would go for login credentials for Apps and accounts which could not be simply re-entered after a phone reset, but I didn’t encounter something like this until now.
Not anymore, but I could.
I did reset the Fairphone 3 once to start over from scratch with less Google.
For some time, I did reset the Fairphone 2 pretty often for testing purposes (back in happier days, when TWRP still could handle phone encryption well and restoring everything was comparably a breeze).