I’m using Etebase as a self-hosted backend for this purpose. It is zero-knowledge, so not even your own server knows about your calerdar, to-do list, and contacts (but it does know when have you edited entries, I think).

There’s an android app for synchronization, and on the desktop, you can run a daemon to expose a standard CalDAV/CardDAV interface for e.g., Thunderbird. The alternative is the Flutter-based client, either on the web or as a desktop app, I think, but I much rather prefer Thunderbird (I have other accounts from work to synchronize, too, and TB lets me look at them in one place).

I’m not sure about the Todoist integration, though. That sounds like it would compromise privacy.

yeah, what we’d really need is issuing ActivityPub Move events for community actors (afaik both Lemmy users and communities are actors on the ActivityPub side, thus communities can be followed / posted to from Mastodon, too). this would allow moving communities between servers to make sure that followers of the old community just carry on following the new community

I guess moving followers of users would behave in a similar way, albeit probably much less critical for the health of communities