Somehow this question has invited … a “what next” reply. You might have asked “Alternatively, go back to FairphoneOS?” There is not an alternative to finding best information before you decide what to do.
You still have your first problem of making a well informed guess of your Android SPL at point of unlock.
get_unlock_ability does not query the indexed date on the phone. It does not tell you that dates inform ability to lock it tells you about device state … is it ok to unlock? No in your case it is not ok to send unlock commands.
When you decide that there is no chance that you will roll back; you could then run that enquiry to check device state. You may get a 0 again, then you will be asking a question like this Week 52, 2025: Development and Testing Updates - #10 by Helight, please follow the answer and reply he gives a nice description of what he did.
My earlier advice is quite simple
I think best you can do is look at Fairpone available downloads and make a guess of the last build you had installed.
Here is the check from that source as a screenshot
To move on
- Don’t roll back
- Best estimate of FairphoneOS Android SPL at unlock
- /e/OS Android SPL is 1st December 2025 (from evidence above).
- Lock the phone if you are certain not to roll back and you get
get_unlock_ability=1