SayCyberOnceMore

Mobile: Fossify Calendar with DavX5 as the interface (both from FDroid)

Laptop: Vivaldi’s built-in Calendar

Tablets around the house: Home Assitant’s calendar (I don’t recall the specific integration, but it was a HACS one from memory)

Why the 1-off release with no further development? (9 months ago)

Interestimg take though, using MQTT instead of XMPP

Radicale - I ditched Nextcloud for it as no-one needed to see a calendar, it’s on their phone…

I also use it to sync a calendar for Home Assistant too

And it effectively backs up my Contacts too.