We don’t know why your updates stopped … with a stable build you should have been offered an Android version upgrade to Android 13 (T) as well as all the security updates you have missed for a year. This alone would be quite a good reason to have a clean install.
I imagine you will want to stay with the official=stable build. The locked bootloader is fine, now you are aware of it.
If you are going for a fresh FP4 install there have been reports that the install script is faulty and a fix was due. This was the last comment Flashing script wrongfully claims the phone is still locked - #15 by Maryn.
The workaround on Windows machines has been to have platform-tools installed as Installing adb and fastboot on Windows PC being sure to do this bit Adding adb path to the environment variables. I think that the bug has not affected Linux machines.