It’s not that it’s closed, it’s more that none of the exiting email protocols support a server which can’t read your email (as it’s all encrypted). They do offer Proton Bridge which you can run locally which will handle all the decryption and local mail clients can talk to that as the would any other mail server.

I don’t know off hand if it supports calendar syncing though.

This was the tool I used. It worked great for me.

Set Immich up a couple weeks ago and I’m surprised how good it is. Their docs included a simple cli tool to bulk import all my Google photos. Mobile app is working great. I’m really impressed with the search too.