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?

Isn't calendar syncing a solved problem? If you set up your own CalDAV server you can pretty much use most any mainstream calendar app you want without having to rely on 3rd party services. Or maybe I'm misunderstanding your request?

Thunderbird is a multi-platform email client, with a builtin calendar functionality. However I do not know if the Android version has a calendar functionality.

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. (E.G. Run Owncloud in a docker on your laptop).

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.

I use DecSync. It saves CalDAV data as local files that can be synced with Syncthing or Cloud solutions. With Radicale you can use Linux clients like Thunderbird on these local files. The Android app is called DecSync CC

Emacs Org Mode would be perfect, but that’s a commitment if you don’t already know Emacs.