IMO you gotta consider the email and calendar functions as inseparable, whereas the rest of the Google bundle can be teased apart. Privacy Guides is perhaps a bit too stingy with their recommendations, but at minimum they give you a lot of food for thought when they lay out their criteria:
https://www.privacyguides.org/en/email/ https://www.privacyguides.org/en/calendar/
I host my calendar on Nextcloud and email (currently) through Protonmail. What makes them inseparable?
Well, you know better than me, because I’ve never not used them together.
How do you, for example, invite people to things? Does your calendar just send an ICS attachment to Proton on SMTP? How do you RSVP for other people’s invites? Do you download the event to your calendar and separately respond in proton? Do you get updates in the calendar app about other people’s RSVP status, or just emails?