I've used Fastmail for years but a year ago switched to Proton. For me the only reason to switch to Proton was that its hosted within the European continent, while Fastmail is hosted in
I would say that Fastmail is the "Ferrari of e-mail" services. It does everything well, or extremely well, especially if you have more advanced setups like wildcard domains.
In particularly, I miss being able to send from wildcard domains. While proton has a thing called simplelogin, it only works kind of seamlessly if you get an e-mail on a wildcard address and want to reply to that same address. Sending from any * domain requires you to make the address via the simplelogin page and isn't nearly as seamless. While you can make some sending addresses (i.e. regular aliases) in the protonmail interface, that's a trap, because once you've made an alias, you can't delete it unless there's no mail related to it in your mailbox anymore (even if you have a catch-all setup; I wonder if it has anything to do with how the encryption keys are setup, but it still sucks).
I also miss both snoozing and pinning mail. Officially, the proton mail apps (1) do support snoozing, but that requires "conversation view" to be enabled. I think the conversation view over groups e-mails too aggressively, and don't really understand why snoozing without conversation view isn't possible. It's utterly annoying. As far as I know, pinning e-mails isn't a thing in the proton apps. There are "stars" but these could have been labels (which also exist). They don't pin the e-mail to the top.
The proton mobile apps also lack various settings which are in the web interface, like access to sieves. The apps are sometimes a bit laggy, especially if you have a lot of e-mails, although there seem to have been some improvement on this end. I also still get double "fingerprint to unlock" requests sometimes.
Then there's theming, which I can imagine is (even) more of an opinion, but I liked the Fastmail interface more than the proton interface. I think its cleaner. Not a particular fan of any of the themes of protonmail.
I left Fastmail just as it added offline access. This was originally my biggest gripe. I might have stayed longer if they added it just before I left.
For Proton, they have been releasing a lot of new services lately. I hope they will spend a year or more, just polishing what they currently have. They did say they will spend some time on polish in a blogpost recently, but haven't really seen the fruits from this yet (or I care about different things than they do?). And I hope I will one day be able to add more domains to my account. Even with Visionary, you only get 6 domains for 6 users, and no way to add more.
I sincerely hope Proton will never add any of the AI nagging , the OP was talking about. If they do, I'll leave the instant.