@freemo I've tried a couple. Etesync is one of the best (it's a paid, open source e2e-encrypted service), but I found it hard to share calendars (e.g. for virtual assistants), so now I use Proton.
Radicale is also pretty good (self-hosted, open source & e2e-encrypted). But since I like to go offline frequently I found it hard to keep my calendars synced.