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 really like that they offer a Gmail migration, including an initial import and _ongoing Inbox sync_. It only syncs the Inbox though, not spam (which is sometimes legit, especially with Gmail) or mail that gets immediately archived by a rule.
I created an alternate domain so I could try them out and perform the switch after a significant evaluation period. Since they have advanced options for figuring out which address to reply to an email with and how, it works seamlessly with gmail and with the catch-all for domains.
I could go on and on. The only thing I miss from Gmail is custom notification sounds. I don't like my email notifications having the default OS sound. Oh and you can't migrate stars/icons for emails. I wish I could do that and convert them to labels, but not a big deal.