This is also how I transitioned to Fastmail. Forward all mail from gmail to your own domain and then slowly unsubscribe or update the email on any that you receive. Eventually you will have most of your accounts and subscriptions moved over to your own domain. I still keep the email forwarding enabled just in case any old accounts send me something.

I did the same but that's not really moving away from google. They still know what emails you receive, what you buy, who you write with, what websites you use. I slowly, one by one, move each website that I use whenever I notice it goes to my old email or I login using my old email.

You can set up pop3 access on fast mail to grab your legacy gmail mail while you transition to a domain and address that you own and control. I still have mine but only get 1-2 messages a month to my old gmail address.

So Google is still getting all your email, you're just never logging in to Gmail UI... Don't see the point.