I migrated away from my main email, it wasn't a Google mail but it was on the providers domain.

First I signed up with Proton Mail and added my own domain, they fit the bill for me, YMMV.

Then I did a search in my password manager and went through those accounts.

Then I just let the old account sit there for a year. Each time I got an email from something I cared about I'd log in and change mail.

It's been a year now, and I'm about to terminate the old account. All I get there now is occasional spam.

I really dreaded this, but all in all quite painless. And next time it should be easier since I now own the email domain.

edit: Forgot to mention I use Thunderbird, so old email I archived to local folders. That's part if why I ended with Proton, their IMAP bridge allows me to keep using Thunderbird.

Nothing. To the contrary things work BETTER outside the google eco system. The way to do it is incrementally. You don't just yolo delete you Gmail day 1. I still have mine, it's just getting almost no traffic today. Start by moving to an alternative email provider. I use proton. Buy a domain so that you can move providers easily in the future and use catch all email. Do a Google takeout and store the backup somewhere safe (I just use two hard drives sitting and home, replicated). Move the thing that you need day to day somewhere else. You can pay for someone to host it for you or self host. I'm self hosting immich for my Google photos replacement. I'm using proton calendar and email for Gmail service replacements. I was already using signal for most communications, but do that. I moved to graphene to get off of android and there are some sharp edges there if you want off Google play. I had to give up Android auto and gps tends to work worse (graphene does support android auto but I didnt like the tradeoffs). Nothing dealbreaking but can be annoying.

For general security, I also use a yubikey for all services that support it, froze credit with all agencies, and added phone support passwords to all my financial institutions.

I've run into one government website that required email addresses to come from gmail.com, outlook.com, or another common domain, and several websites that won't let you change your email address once registered. It also makes it really confusing if someone needs to share Google Docs with you. So I've moved as much as I can off of Google, but some stuff will linger forever.

I use Fastmail and the main difference I notice is less effective spam filtering — it’s good but not as great as Gmail was.

Overall it’s been an acceptable trade off and I’m glad years ago I switched to a custom domain for email so I can have portability.

Personally, I deleted everything I could but kept the Gmail account for a couple of years with a forward to my new account, and after that, I also deleted it. Google Takeout is a very useful way to quickly create a backup of everything Google.