- there was no way to move the "root" domain name away from Google Workspace while the account was still in service,
- after deleting the account, any access was definitely lost (I spent a lot of time on the phone with them, but they provided no help),
- the lost domain name was never "released", it remained officially under my ownership until the very last day of its registration period, even though I had no way to act on it.
- when I called that out a bug, they firmly replied that no there was no bug...
So, the lesson is: never use a domain name important for your business as the root domain name of a Google Workspace account, unless you really are sure that you will never need to move it to another registrar nor to delete the account.
Moreover the domain did not belong to my GoDaddy account (which makes sense, as I created the GW account and acquired the root domain several years before I signed up to GoDaddy). So maybe it was a hidden GoDaddy account, something like a service account for the exclusive use of GW internal tools?