Tom,
I tried to run initdb after re-installing pg 12 using postgresql-12.10-2-windows-x64.exe.
But the runas I'm using to execute it as pguser seems to be swallowing all the output, so I can't see any errors.
I was able to run pg_checksums and get those enabled.
Is there anything else I want from initdb?
Or can I skip that?
Nonetheless,
your path forward is clear: use pg_dump (or better pg_dumpall)
and then load the output into a freshly initdb'd v12 installation.
It'll be a bit slower than the pg_upgrade way, but it'll work.
regards, tom lane