When I left Gmail for FastMail, I imported my contacts and calendar from Gmail, started fresh with the email. Nothing is forcing you to delete your Gmail, you can always go back and reference it. You can export your mailbox from Gmail and import it into a local mail client, I just didn't bother.
For each new service, I'd sign up with my new custom domain email instead of my Gmail. I'd continually check the Gmail for emails that I cared about and would update my email for those services/mailing lists as they came in. Over the period of a couple years.
I maybe move one email over a month these days, so we're just about done. With a custom domain, I'll never have to do this again.
The best time to start is a few years ago. The second best time is now.
For each new service, I'd sign up with my new custom domain email instead of my Gmail. I'd continually check the Gmail for emails that I cared about and would update my email for those services/mailing lists as they came in. Over the period of a couple years.
I maybe move one email over a month these days, so we're just about done. With a custom domain, I'll never have to do this again.
The best time to start is a few years ago. The second best time is now.