Something similar happened to me with one of my rarely-used Gmail accounts, got prompted for the security question after it took me until my 3rd attempt to get the password correct. I had set the answer to gibberish some 8 or 9 years ago, and had zero way of recovering the account. Found lots of people with similar issues on the Google product forums, all answered (not by Google employees, of course) that it was completely unrecoverable.

I wrote off the account at that point and migrated a few accounts attached, but 2 weeks later the system let me enter a phone number to be able to log in.

Moral of the story: pay for support if you want it.