This person had similar issue: Error during 14.0.12 > 14.3.6 upgrade - #2 by unique
Not waiting for background migrations to complete before starting next upgrade is a problem and well documented in Gitlab upgrade docs. I don’t know about the manual update with SQL, search the forums for bigint, there are plenty of posts already one of which I’m pretty sure will have a manual way around this I seem to remember seeing something like this before on this forum.