Same here. When you say your own email, do you mean Fastmail? I love Proton and they got me and my SO largely off of Google. Fastmail is just so good, I’m going to migrate everything there eventually.
Once you have more than one mailbox then its kinda the only way. So apple mail, thunderbird etc. And then you mostly get some quality provider mailbox.org, fastmail, infomaniak or whatever but its all the same.
Not sure what they mean, but at the very least I think you should have your own domain. So that you can migrate without changing the email address (and having to ask all your contacts to update to the new one).
I assume they mean they run their own server now.