Thank you for the OS listing, bookmarked. What I look for is similar to that wiki page, just not listing OSes but listing issues of Fairphone OS 11 respectively 13.
Backup does not help against lost functionality (i.e. bugs in new version) 
Concerning data losses: I had a backup, but only when trying to restore I detected that the backup missed contacts of one of the 8 accounts – following Murphy’s laws, the account that was discontinued in the new Android version and thus deleted by intention during the update process. Would have been user friendly to notify for this intended data deletion in advance, so I could have moved contacts to another data pool. For the other losses, well, I do not know of a convenient & reliable way to backup & restore local application data, but I am happy to learn about it 