Never give up. Today I succeeded in flashing my CM3 (maybe it was some thermal past that was short-circuiting something, that I managed to clean).
Since my last back up was when I moved from CM1 to CM3 (=3 y ago), I had to downgrade the CM3 to 7.95.1 Restore the back up, and update to 9.2.3. Everything was smooth this time.
Then I spent some 2h tagging again watched movies and shows and I am now back to "normal"
The only thing is that I have already a RPi4 on the way, but at least I had plenty of attempts to rehearse how to flash, back-up, restore, etc...
Again thank you for the support.