You have real customer support (a person) who gets back to you in a timely fashion and is always helpful. The android app and web app are intuitive and easy to use. I switched from Gmail to Fastmail a few years ago and couldnt be happier and it's cheap.
My stack is:
Custom domain (current registrar: namecheap)
Fastmail for email, Calendar & Contacts hosting with DAVx5 & Fastmail App on Android,
Netlify for DNS & static website hosting
Switched everything over from Dreamhost after 14 years or so in 2020, and the only thing that would make me happier with Fastmail would be for them to have global holiday calendar syncing (This is the only reason my Fastmail account is linked to a dummy google account - to sync holidays!)
I get to use email aliases, catchalls and a mail system which can automatically file mails before I have manually written a single rule. Off the top of my head, the aliases, the sensible (vim-like) keyboard shortcuts (g for search folders, j-k for navigation, etc.), built-in caldav and carddav support are the things that make life easy on a day by day basis. Notes are about the only thing I don't use, but my encrypted Joplin notes are actually synced to the Files section on Fastmail via WebDAV since Fastmail also offers you decent (At least 10GB if you have a standard plan, which you need for a custom domain) of WebDAV accessible storage along with email...
15 year Fastmail user here. Fastmail has been great, no downtime, and I don't really think about it at all. Good web client and you can do filtering if you need to.
They've just been super reliable and mature for me. No huge updates that force me to adjust. Interface is nice and clean, uptime and service super reliable. Everything just seems to work for me (aliases, forwarding, filters, good Android app), and the pricing is within reason.
I actually use their web interface and official Android app. I'm surely no power user, but have used various providers over the years until I settled with Fastmail (seemingly for good). It's difficult to decide whom to trust with e-mail but to me they seem trustworthy. I still like to have my own domain, so I could in theory take it with me, if I ever started being displeased.
In general: Paying for a service (especially e-mail) is invaluable (my data not being mined), also for potential account recovery (didn't use their customer service often, but when it happened, it was decent).
I can't comment on ProtonMail but especially with e-mail, a proven (long term) track record is gold, and Fastmail has it.
addendum: upon rereading it almost sounds like a Fastmail commercial. I'm only a satisfied user without any further connection towards the company.
I'm not sure if email can ever be secure, or put better if I could ever set it up and keep it secure. So the jurisdiction issue wasn't a big factor for me (although I'd prefer Switzerland over Oz's warrants with gag orders).
At the time when I evaluated, FastMail just worked, and their standard support remains excellent. IMAP works great. The ProtonMail bridge thing put me off to be honest. There's not too much else to it.
Either way though, both were preferable to Gmail, which I wanted to drop along with all Google stuff if I could. And being lazy/just wanting it to work, I chose Fastmail.
I think I’m trying to decide on the tradeoff between ProtonMail for privacy reasons and FastMail for good support. It’s not clear to me what implications there are for the bridge thing or their location.
The privacy stuff has been discussed to death elsewhere. All I can say is compare the setup instructions for either service on desktop and mobile (especially if you're on iOS). Maybe calendar and contacts sync too, if you care about that. If you can live with the ProtonMail way of doing things, go with them, else FastMail.
I switched to fastmail from migadu last year. My killer feature is being able to use a custom email address per company without having to provision one (ie, a catch all that you can do send as x from).
I left migadu because they repeatedly demonstrated they were neither professional nor reliable enough to host my email (that is bargain basement pricing for you). I looked around and trialled several services on my other domains; I settled on fastmail as they had a proven track record with recommendations from people I trust who know email, every feature I wanted, and decent pricing.
I recently did a big survey of the mail hosting market. I settled on migadu. Have used them for two months now and am very happy with them. Consider checking them out.
Between those two, proper IMAP support (including IMAP push) is the big thing. Webmail generally drives me nuts for anything beyond stopgap usage, and while ProtonMail has a local IMAP adapter thing it’s a step too involved for me.
IMAP support is incidentally also why I don’t use Gmail. Their IMAP implementation is spotty and notoriously bad with Apple Mail, whereas the experience with FastMail is flawless.