Offline secure container
If PD functioned like dropbox [DB] with local offline file storage that synced when changes were made that would be ideal. To secure the local data implement a secure storage container [VeraCrypt type] the OS could mount the container, then after decryption/opening use that as the local secure copy. Canceling this [mounting of the container] would effectively pause all sync operations.
Possibly add config. options for re-enter password for local container every X minutes/hours with or without data changes in container, at logon [close the container at log off], after resume from standby/sleep/screen saver activation.
Have a button/hotkey that would close/lock the local container without the machine having to standby/sleep/screen saver, on demand.
And for Gods sake, have configuration options at the file/folder [descendant folder] level for specification of, upload only the parts of the file that have changed, or upload the entire file on save/close/WiFi [non data metered connection]. Maybe even have a scheduled sync during X time to Y time [0000-0400hrs] or on demand [button/hot key], sync immediately any file changed [part/whole, metered/not metered connection depending on config described above] under X kb/mb.