I'm in the same boat. I think part of that is Proton is spread across a huge suite of products and features, whereas Fastmail is specialised in one.
> Fastmail is specialised in one.
Sadly untrue since they added calendar. However I'd would say the email service and support remain excellent regardless.
It feels like Proton is trying to build a solid Europe-based alternative to Microsoft 365, which is necessary but also very ambitious and expensive.