Google Compute Engine SSH - different username after switching to OS Login
I asked this question here and got a clear answer.
Why is the username different between OS Login enabled and disabled?
OS Login ties your Linux user account to your Google identity so that you have a consistent username, UID, and other posix information, in every VM you log into.
When OS Login is not enabled, the username is set in your SSH key which has no tie-in to a user's Google identity. The username is generated to be convenient, and none of the OS Login-specific concerns apply.
And can I influence the username when OS Login enabled?
The posix information is immutable for consumer identities (non-G Suite users).
So no, I can't, because I'm an individual and not an organisation.
In the end, I solved the inconveniece this caused me by simply recreating the server with OS Login enabled - that of course is one of the strengths of Compute Engine.