I'm been in this process for the last two months (because there are so many sites to change my e-mail address). I went with Gmail originally so I did not have to change my address everytime I changed ISPs.

So my first step was to use my own domain name so that as long as i keep renewing that annually (it's a small cost) then I keep my e-mail address (even if I change actual e-mail providers).

Next was to find an e-mail service I could point my domain to. I also needed one where I could download my e-mail to my desktop computer for archiving and reference, eg. I'm using BetterBird, but ThunderBird is another option.

So finding a mail provider is interesting as most that let you use your own domain name (custom domain name) are not free. One option was paying a small fee to by actual domain provider and use their service. I went though for ProtonMail in the end as I was already paying for their VPN service, so the difference to upgrade to the "everything" account was not that much more and scored me 500GB of online storage too (I pay separately for Bitwarden password management, otherwise that would be another plus). This allowed me to use my custom domain name (unlimited addresses), download mail through their bridge, etc.

So the real challenge really was finding a suitable mail service. I can switch easily in the future as I just point my domain name to the new mail provider, and never have to update my mail address at any sites again, and all mail is always available on my desktop computer.