- 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.