First start using personal domain so that when I switch away I can do it without having to update all contact details. I’ve already been doing this.

Then after some research, switch to a paid email provider. I’m thinking of ProtonMail or Zoho at the moment but I need to look into it more.

It’s going to be painful because I used to use Google OAuth whenever it was available in the past. But I would like to start having my eggs in different baskets.

EDIT: re: "whats the line"

There is no concrete line. It's mostly me getting old enough to worry about not losing access to a lot of services on algorithms' whim and I keep seeing these posts on HN (Google or not). I know the chance of it happening is slim, but I think the damage would be big enough to diversify away.