It will be a long process no way around it.
I would first acquire your next domain, are you planning to subscribe to protonmail or tutonota?
First of all make a list and comb through your folders + inbox and try and find all the most crucial traffic that needs to be updated to your new address.
You will likely have to visit ton’s of sites (at least every site you linked to your gmail account to login, and or recover your password / + 2FACTOR AUTH [!] from.
What I suggest doing is when you decide to go through with this.
If you don’t already. Start using a password manager.
For ease of use and pretty good security I would personally recommend using bitwarden.
It is available for all platforms and syncs it’s content.
Index all the accounts after and during the process of changing up your e-mail for all the accounts you will do this for.
After this you will have all of the data in bitwarden.
10+ if you create 40+ character number passwords for each account while you also change up the email.
This will greatly decrease the chance of one of your accounts being hacked or simply walked into after a breach.
Keep the gmail account btw. You likely will encounter from time to time. That there was still this AND that and damn also that. Still requiring you to open up gmail once in a while.
You don’t want to get locked out.
What I wish I did when I moved form gmail towards protonmail at the very start was combining it with either
or https://anonaddy.com/ from the get go.
This prevents companies and whomever else from discovering your real address behind the service these apps / companies provide.
Thus further protecting anonymity.
Good luck it will take a while.