The problem with separating Calendar + Mail + Contacts is that they work best together. Although to be far I am not aware of an open-source system that effectively combines them.
Calendar event invites an updates go over mail. So you want your calendar application to automatically be able to get those. Also options like “automatically add invites from contacts to my calendar” is an awesome feature. Contacts can also be used for spam filtering (although this integration is a bit easier to do externally).
So currently I am using Nextcloud (self-hosted) although I don’t really like it because it is pretty slow on my low-powered VPS. But even still it doesn’t actually have proper email integration. There are bugs open and slowly moving but I’m still using Thunderbird to process most of my calendar stuff.
Not to mention JMAP which is slowly progressing which would be a huge improvement, especially for mobile clients. It also combines these three services.
This is the advantage of decentralization over federation. IMHO the fact that Lemmy is only federated really hurts it. Not so much for user accounts (in theory these can be backed up restored and moved. Not ideal but not awful) but in that communities are tied to servers. When the server a community is on goes away it is hugely damaging to that community.