Very likely ME/CFS.

Also, you apparently don’t want to hear this, but a huge percentage of sufferers of long COVID had no acute symptoms, have no positive test, and otherwise have no reason to believe they ever had COVID except a) chronic symptoms consistent with long COVID b) which came on in 2020 or 2021. Nothing about “I wasn’t” (acutely) “sick” or “gradual onset” sways me in the slightest away from the possibility that this sounds very much like long COVID. Both are entirely consistent, even typical, of long COVID.