One of the very few apps I actually paid for. The mental peace that comes with not leaking my data to yet another calendar application alone was worth it. I find this business model lot more sensible than demanding that open-source developers should eternally martyr themselves. You can see the effect of this in the feature set of these applications.
If you are the semi-paranoid kind, the following stack of opensource apps will enable you to keep a calendar/contacts/tasks in sync between Linux and android without an internet connection.
On Android Side
1. Simple Calendar
2. Dec Sync (Fdroid)
3. SyncThing
On Linux Side
1. SyncThing
2. Evolution with Decsync plugin
What Decsync does is keep your calendar/contacts/tasks data in a single folder in open format. This can be read on GNU+Linux side with any app that has a Decsync plugin. This folder can be backed up using whatever is your backup solution (For those who are curious - I do a encrypted incremental backup with BorgBackup (Vorta GUI) and then upload to cloud using Rclone).
Decysnc also supports syncing RSS feeds between applications, although I have not been able to find a working Linux RSS app that has Decsync plugin.
It's nice that a flow exists for your use case, but when I see things like this I'm always puzzled by the use case in the first place. The only reason I use a calendar on my phone is to sync my schedule with the wife/family. We put things on a shared calendar. I use it to capture appointments and check if we have commitments on a given date. Otherwise I would never use a calendar on my phone. So syncing between accounts over the Internet is a hard requirement.
In that case why don't you sync your calendar with Nextcloud directly using CalDav?
I use DavX5 for this and it works great, actually i think DecSync is a fork of DavX5.
If you are the semi-paranoid kind, the following stack of opensource apps will enable you to keep a calendar/contacts/tasks in sync between Linux and android without an internet connection.
On Android Side
On Linux Side What Decsync does is keep your calendar/contacts/tasks data in a single folder in open format. This can be read on GNU+Linux side with any app that has a Decsync plugin. This folder can be backed up using whatever is your backup solution (For those who are curious - I do a encrypted incremental backup with BorgBackup (Vorta GUI) and then upload to cloud using Rclone).Decysnc also supports syncing RSS feeds between applications, although I have not been able to find a working Linux RSS app that has Decsync plugin.