That's why you should have email on your own domain. If they lock you out you can quickly-ish migrate to a different provider without losing your email address. Not a solution but definitely softens the blow.
That is my arrangement and I've had it in place so long I don't even have to pay the $5, I have a free private GSuite on my own domain (yes they offered this to everyone 10+ years ago). What I realized today though while reading this story, is that my DNS account is backed by this same GMail account. What if I'm attacked and someone locks out my DNS account at the same time they take the sort of criminal actions in my account that that would get Google to shut it down? I could lose my domain and all my email forever. I think I'm moving to Fastmail.
i actually think it's useful to create a flow graph of all your most critical accounts, and how you might recover them if lost.
If you find many of them flow back to an account you cant control and may be shut down for arbitrary reasons, may want to reconsider your recovery/hierarchy.
yeah I have ProtonMail with 2 custom domains. It's decent I have gotten blocked from sending emails to Google custom domains. Total bullshit IMO, I think they do it on purpose.