Interesting. I have a daily driver FP3+ where I installed 0.x-dev based on Android 10 years ago (can’t remember exactly what version, probably 0.14). I locked the bootloader and used it ever since.
I updated/graded through everything. Always about three update cycles after an Android version change I upgraded through sideload without bootloader unlocking or any data wipe. Just last week I upgraded to 3.3-a15-community which I am now happily running without problems. The update process has been amazingly stable and I never had data loss during upgrades.
So, naturally, I wonder what the problem upgrading the FP3 should be.
The only major hiccup I remember was from A12 to A13 or maybe A13 to A14 the system UI would crash fatally when opening a media player in shelter because of some problem in the media notification. That was fixable through adb, though.