You can have you gmail forward to your fastmail account. This way you leave it up but migrate over as you can (you won't miss updates, password reset emails, etc).

Once you have your new domain setup on Fastmail, ProtonMail, etc., you can set up a “vacation responder” on gmail so anyone emailing you there will get an email with any message you want to cajole people into updating their address book.

Before I used a password manager I just looked through my Gmail and logged into every site I had an account with and changed the email address to my personal domain one.

This took about a week of focused effort.

Imagine one day you accidentally like a wrong Facebook post and get "canceled". Poof your google account and you logins with it ...

I both have a custom domain email (mostly for my own work) and a hey.com account. I did not try not move everything from gmail to one or another email as it would be painful. However, I'm moving things time to time and most of my new subscriptions are using either my own domain or the hey domain. In a year or two, I expect to start to redirect all remaining gmail emails to my new main email.

I have fastmail set up so that it continually imports new mail from my google accounts, and if I reply from the fastmail interface it replies using the email I received the message on.

It's a very easy first step. Then you can migrate things over one by one as they come in without worrying about missing things or switching between interfaces.