I’m fine with getting invites or something, I don’t quite understand why it has to be email. Why not just a link? When I share a google doc, it’s not only over a email service of the same vendor, it’s just a link I can send anywhere. That’s the inextricable link.
edit: they’re just .ics files anyway aren’t they?
I looked up planners out of curiosity, but it just gives me paper planners or todo-list apps. Is there any specific service you were referring to?