AFAIR none of those allow sending and receiving calendar invitations, that interfunction with outside services (eg Outlook.com, Gmail, Apple). Proton does. Send a calendar invitation from gmail/outlook/Apple to a Proton user; they can accept it and it will automatically add to their calendar and send back an acceptance email that gmail/etc understands. Vice-versa also true.

I trialled almost all of those before I ended with Proton, because it was the only one that felt like they actually tried to get calendar and email functionality on par with major (non-privacy focussed) service providers, and in general was the most polished.

They are all fine if literally all you need is simple email though.