Yeah, email isn't private, but for me it's usually that I don't like my host reading all of my mail to build an profile on me and selling that data. Individual emails in isolation aren't a big deal, but seeing every email and what company or agency sent it is as problematic as the content even if you encrypt the mail content itself. Emails that I sends I always assume are not private, but that's a separate issue, IMHO. There's a lot of private information like what protected classes I am part of, political leanings, places I shop, etc., that can be gathered simply from who sends mail to you and who you send mail to. This is why I self host for most of my email.

That being said I still use gmail as I need a backup option and I use it for things where I don't want the junk sender to know my domain and spam all of my accounts.

But Proton is really not much more private than Google in several scenarios given their CEO's stance on several sensitive subjects and willingness to give data on protected classes, journalists, etc., to hostile governments, as an example. They do say they don't sell your data to ad companies, at least. I don't know Fastmail at all. And self-hosting is not something I'd recommend if you don't want to put a lot of time and effort into it. Lots of issues come up like blacklisted VPS IP addresses in addition to the setup itself.