Edit: I seek a server-less solution.
The only thing I can see is Linwood Flow but it looks like it’s a really, really long way off before becoming a primary calendar app.
If it could just store all calendar content in 1 local file, then that could be so easily auto-synced across devices with !syncthing@programming.dev. Does anyone have any leads?
Hmm, I just now learned about Fruux, in this Reddit post…
Give us an example of what you want as the end result - what devices you have, are you sharing calendars with someone else, etc.
My best answer is to run a calendar server on some machine and let your calendar sync to that whenever the devices are online on the same wifi simultaneously.
Alternatively you could run Tailscale on the devices which would provide a secure mesh network, eliminating the need to be on the same wifi - so long as they’re online they can sync via Tailscale.
Tailscale even has a feature (Funnel) that will route specific internet traffic into your Tailscale net - this would eliminate the need to have Tailscale on every device. You could host a calendar on a laptop (say Nextcloud in Docker with Tailscale), enable Funnel only for the calendar port, and apply security in Tailscale so only you have access.