You already know much more about this than I do. Thanks for sharing!
I thought of backups like this:
File base backup
Use this after partition layout changes or for a backup of data if needed (phonebook, messages = the .dbs, apks and configs).
I think of running rsync on a rooted device with a rooted adbd. Have not tried that yet.
Block based backup:
I thought dd would work on everything as long as each partition stays the same. As a starting point after every upgrade (new Android/FP OS version).
But I guess if this would be that easy, there would be a working default backup app 
I just think it could be useful to have a “non commercial” (=no appstore) build backup that everybody can use and talk about as “build in tools”. Other tools might be easier but this would be the lowest level to catch possible problems (like a broken backup app or dying storage).
How does the backup on current Nexus phones work, I wonder?
This is what I found (never used ):
DD backup app
rsync app