Note that while I'm happy to deal with smaller businesses, and happy to spend money on them, I am still pretty wary of advertising-based businesses. It is a slippery slope of doom, and I think it's ideal to try and stay off of it.
Can you elaborate on this? Are there some privacy issues with Fast Mail that don't exist with ProtonMail?
ProtonMail encrypts your email storage in an end-to-end manner for client access and storage. (Obviously, unencrypted mail sent to or from other servers could be intercepted.) But their goal is essentially to be mathematically unable to view your email. Of course there are tradeoffs, things like IMAP don't work (without some sort of relay), for example.
Essentially, FastMail makes a point to not violate your privacy, ProtonMail tries to make it so it can't violate your privacy. I lean towards FastMail because it's 'good enough' on privacy, and has a lot of powerful features, but if you're looking to run afoul of state intelligence agencies, you might wanna lean for the latter.
Is this just TLS + block level encryption like LUKS? If so I would be surprised if FastMail wasn't offering the same. But maybe Proton is offering something else at the client level? It was my impression that the big differentiator was Proton Mail's infrastructure was all co-located in Switzerland.