I pay for both, use them for different things, but to be clear, fastmail does not have the encryption features that ProtonMail does. Not a direct replacement if you’re interested in the privacy aspects.

What encryption features? GPG works with any email provider.

Any specific improvements?

I ended up switching to Fastmail after it being recommended here ages back, and it’s been fantastic.

I’ve also used Google Workspace, O365 or whatever MS calls it this week, and Proton as well as self hosting mail in the past.

I'm honestly shocked by how long I've been using Fastmail without having a single complaint to talk about.

Usually after using a tool for an extended period of time, there's gonna be some annoyance that you just have to learn to deal with. After five or so years of me using Fastmail, I can't think of a single thing that actually bothers me.