I was using the UI and not CLI when I set mine up, but I agree is odd that CLI didn’t work. Perhaps the --update-env-vars was hiding the fact some previous env vars had been set which were breaking things?
When you switch to a custom domain hopefully all you need to do is change BASEROW_PUBLIC_URL=your new domain. However if gcloud is proxying and changing the Host of the requests in any way then you might also need to set BASEROW_EXTRA_ALLOWED_HOSTS=https://[SERVICE URL].a.run.app or whatever the host is that the requests are coming from