I successfully extricated myself from Gmail to ProtonMail, only to be getting dragged back to Office 365 due to ProtonMail not having a working calendar and FastMail not supporting calendar sharing (to non-FastMail users) or delegation.
proton is working on calendaring but it still has a long ways to go.
So we're just going to pretend that iCloud doesn't exist, then? Is that it?
It's fine for simple use, but without the ability to create calendar invitations it's far from the competition. (To say nothing of sharing e.g. free/busy with my work calendar or delegation.)