What about the fact it’s a US-based product so you’re under forced Five Eyes data collection?

Almost every email is going to or coming from US servers anyway. Better to treat email as insecure and use something else for more secure communication.

Correct. I mean if you send from ProtonMail to GMail (which will be 90% of private conversations) you're an open book anyway. And if you want to be sure then there's PGP et al.

And how does Protonmail help when Switzerland has signed the Mutual Legal Assistance Treaty with the US? If they need anything, they’ll get it.

Fastmail is not a US product, it is Australian. Australia is part of 5 Eyes though.