I moved to Fastmail last year and it's been entirely unremarkable which is exactly what I want. Mail in and out works, it's reliable, I have my custom domains.
It really depends on the level of privacy you're going for and what features you want. For me I needed custom domain support with catchalls. The only other requirement I had was to not be Google. I debated between Fastmail and Proton for a while (Fastmail for features/price, Proton for the "better" privacy.) Ultimately I ended up on Fastmail because I would have had to pay for a higher than necessary account at Proton for what I wanted.