I’ve been a Fastmail user for years, having left Gmail. It works great and have nothing be but praise for them. I use my own domain with them so if I decide to leave it’s not an issue worrying about updating people with my new email.
I don't think that's true. Some years ago I did a free trial with them (did not pay anything). More recently I decided to actually sign up (for a paid account) and the email address I used for the free trial years ago was not available. I eventually got that username only after contacting support and giving them the date on which I started that free trial, to prove it was me.