OK guys, an update from my side. I have been short on time due to new job and preparations for moving.
I also decided to go for it and use your generous cash donations to buy a used FP3 for better testing, rather than having to use my day-to-day phone and risk loss of data. I got the new phone and then the fun started.
The person I bought the phone from did not unregister from their Google account before wiping the phone. Most of you will know about Google’s Factory Reset Protection (FRP). To keep it short - I was locked out of the phone and could not even do the OEM unlock steps I have advertised on LineageOS.
So what to do? I started hacking on the phone seriously, meaning I accessed Qualcomm’s Emergency Download Mode. Details on how to do that are documented by other community members. I read out the application boot loader (aboot), analyzed where it looks for for the OEM unlock permission and used this tool to overwrite a single bit of the config partition - and the device was unlocked. I then flashed LineageOS successfully per normal fastboot procedures. LineageOS does not care about FRP.
To make the long story short: FRP and OEM lock can be bypassed on any Fairphone 3. If you ever buy a used phone like I did and run into the same problem, feel free to PM me and we can try to sort things out.