My gmail address was also almost exactly 20 years old when I migrated two years ago. Here's how I did it:

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1. Register your domain (if you're doing that) and get fastmail set up -- I remember feeling a physical discomfort clicking "register" on fastmail, it felt like such an insanely impossible thing to do.

2. Set up the IMAP link so that anything sent to your gmail gets delivered to fastmail. Doing so also allows you to send email from your gmail address (with valid spf/dkim) if you want to.

3. Import all your old mail using fastmail's import tool, which Just Works.

4. Set up a vacation autoresponder in gmail that responds only to people in your contacts with a note telling them your new address.

5. Set up a label and filter in fastmail for anything that was addressed to your gmail, so you can easily see what is still sending you email on your old address to assist with migrating services.

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It worked a charm. I was completely convinced of it within a week, long before the 30 day free trial ran out. I have been an immensely happy customer since then. Could not imagine going back.