I have used Fastmail, and it is good. Currently I'm using my own domain on iCloud+, as I'm already paying Apple for the right to do so. But that's the good thing about owning my own domain...I can take it with me.
Protonmail is very expensive for what it is. I don't care about their privacy (it's email) and I don't need anything but an email service that can support 3+ domains.