I hope you don't mind me ranting a bit about custom domains
> Custom Domain Protection for Cloudflare Registrar, available on the Enterprise Plan, protects your organization from domain hijacking with exclusively out-of-band verification of any changes to your Registrar account.
This is what keeps me locked into Google and other services. I just can't trust my custom domain, if I'm targeted by any semi competent attacker it WILL be hijacked. That you're offering this service only makes my suspicions stronger. I want to use your services but that's a showstopper. It's not your fault, of course, all registrars face the same issues. You need so many different factors to make the process secure it's not even funny, and you said it yourself: "That, obviously, doesn't scale".
A few years ago one of my customers domain was stolen by contacting the registrar support (one of the big ones, always recommended around here). They even had a scan of his passport. With so many data leaks, even from your own government, how do you even protect against these kind of things? His life for the next few months were living hell.