Help finding an Android CalDAV sync app for an airgapped tablet that NEVER connects to the internet

My tablet and home network are airgapped and NEVER touch the internet, so please don’t suggest some variation of “connect your tablet/nextcloud server to the internet”. I cant do anything about this.

I have nextcloud setup and working great as a file syncing tool between a couple of computers and my tablet, but a huge problem point is calanders. It seems everyone recomends using DAVx5, but DAVx5 relies on Android sync framework. which aparently refuses to do any syncing without internet access.

So I need a caldav app that lets me sync between nextcloud and my calendar (Etar) without relying on the Android sync framework.

Or maybe some other hack/workaround that would let me just make a calendar event on my tablet and have it sync with my computer and vice versa. I’m tired of importing and exporting isc files every morning and evening, my tablet is connected to the local network, i should be able to sync calendar events automatically.

Other apps I’ve tried that dont work are:
OpenSync (fork of DAVx5)
DecSync CC (does not work with nextcloud),
ICSx5 (subscription only)

Any other suggestions for connecting my tablet calendar with nextcloud calendar?

The tablet is a samsung SM-t290 using android version 9

The simplest I can think of while not going through internet would not need an app, if your apps can use output and input : can you export your calendar as .ics file ? And from there import it back in the destination app? The export extensions are not always of the compatible kind, but that’s the only lead I can think of.