With the recent integration of the jex import in the mobile app, there is now the flexibility to be able to easily transfer data to and from both desktop and mobile devices. However, if you want to transfer data between 2 devices while sharing the same sync source, the only way to do this without essentially deleting and recreating everything (which requires a total resync) at the sync source is to download the contents to the new device via sync.
Unfortunately syncing to a new device can be very slow for various reasons for a large collection of notes, whether it is syncing with OneDrive / Dropbox which is throttled / unoptimised, or file system sync which may be very slow on mobile, depending on which version of Android you have.
The reason for jex import not being a viable solution to this is that all of the note, tag and potentially resource ids are replaced with new ones when doing an import, and this is logical behaviour for an import feature. However, could a slightly modified version of the import jex feature be implemented as a separate menu option for desktop and mobile, which will retain the existing ids (and updated timestamps, if not retained already), and therefore will not trash your sync source when syncing to an existing source? I propose implementing it essentially as a 'create new profile from backup' option. This way you do not need to worry about dealing with any conflicts, because it will create a brand new profile, but with all the note data matching to the existing profile it was exported from.
It's perfectly reasonable for the very first sync of a large number of notes to take a long time, but it's not ideal to have to wait a long time for every new device setup. I think implementing this solution could provide a quick win to resolve the latter.