Truly decoupling yourself is hard, and you'll probably be finding old accounts and dealing with them for years, but getting 95% of the stuff moved over the next year or so will likely give you a lot of peace of mind if you're actaully worried about it.
FWIW if you're actually worried about it, you should also make sure your email is through a domain you control (even if just pointed towards a service, or even gmail) and that domain service is not linked administratively to the same account that you use for email (e.g. don't register domain through google and then point at gmail), so worst case you can change the destination service by changing MX records and just swap all your email at once to a new handler (or if you're a masochist or as a last resort point towards your own mail server you admin). Just make sure it never expires (in the end, there's always something to worry about, you need to find the risk you're most comfortable with).
2) Buy mail services from fastmail
3) Set up forwarding in Gmail
4) Every Saturday you go through your inbox and change the email address on accounts important to you, and unsubscribe from the rest.
And you should still log in to Gmail of course, to avoid account shutdown. Also, Gmail doesn't forward what it considers spam.
I also found their search (which I rely on heavily) to be a joke compared to gmail.
I still have the old gmail account, but almost nothing of value goes through it.
The key is the domain, you don't even need to go away from gmail, but own your domain.
Don't you still have to worry about losing the domain?
Transitioning to a new email address isn't really that bad. I did it about 15 years ago. Whenever you log in to a site and you notice that it's using your old email address, you go into your profile settings and update it. After about a year you'll probably have hit 90% of your meaningful accounts.
I keep seeing fastmail here. Is that the best alternative right now?
I've _still_ got a few places that insist sending mail to Gmail (mostly sites that don't let me switch email for some weird reason).
I got my own domain, set it up on Fastmail, imported all my old mails from Gmail and set up a permanent forward from Gmail to Fastmail.
Then I set up a filter on Fastmail that shows all mails that were sent to my Gmail address and slowly went through that list and switched from @gmail.com to my own domain.
Now I'm the customer for my email instead of the product and I 100% own my own domain so no single company can take it away.
There's your problem fishbulb.