Unable to Delete Synced Items on Onyx Account

I cannot seem to delete the synced reading data of my library or notes.

In using the Boox Assistant App, it provides no such option to delete already synced notes or reading data. The only option it provides is the option to disable sync, but that is not the same as deleting the synced data. The previously synced data is still present after I disabled all sync. (As of this time, it appears the Boox Assistant App is the only way to view synced content on the Boox server)

I do not want that data stored on the cloud. If I try logging out of my Boox account, it deletes all notes from my local device. And there doesn't seem to be any way to preserve them locally without syncing them to the cloud. I only want to store my data locally on my device and not on the cloud. And for any data that I do sync to the Boox cloud, I want to have the option to delete it without deleting my entire account or the in question on my local device.

If anyone know of any solution, please help! Or Boox, if this is not already an available feature, can you please make these options available?

As a user, I want to have control over what I want to maintain on the cloud.

The device I have is the Tab Mini C.