So at the moment my Partner and I use a shared Google Calendar that I setup for our events/schedules. We use one calendar and prefix our name to the event so we can differentiate. EG:

Name1 - Office

Name2 - Dentist

Both - Dave's Wedding

I've managed to remove Google from most things and was wondering what privacy/security focused alternatives there are.

I was looking at Proton, but it seems it doesn't play nicely with HA, due to it being encrypted. I also can't test without signing up, so not helpful.

I could self-host if there's a suitable option.

I guess I could go Proton still, have HA in read-only, then use the native Proton App for adding events, just helpful to have the option to use HA.

In addition, is there a better setup for multiple people & calendars? How do you do "both" attending? Do I actually need 3 calendars?

In an ideal world, Proton would release a native HA app!