Same setup as the comment you're answering 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.