It doesn’t answer your questions about calendar and contacts, but you might still find it interesting to take a look at this project:

https://maddy.email/

https://www.theregister.com/2024/08/08/dot_internal_ratified/

Just curious: If you’re willing to use Discord, why not Google?

Imo not really noob-user friendly.

In what way? It would make it entirely invisible that the archive file isn’t just a normal folder, it would be possible to use it just as if it were. What would be unfriendly about that?

The operating system could mount it as a virtual drive, then all its contents could be used directly just like any regular folder.

You might find this project interesting:

https://maddy.email/

As already mentioned several times, selfhosting a mail server is not recommended unless you’re particularly interested in hosting a mail server, but with that said, you might find this project interesting:

https://maddy.email/