Found another solution. I spoke with the Astell & Kern folks since I have an A&K DAP. They suggested trying Commander One software from eltima.com. There is a free version I downloaded, and it seems to come with a free 14 day trial of the Pro version. It is not clear whether the Pro version functions are needed to access DAPs. Guess I will find out when the 14 day trial period is up :-).
The first two times I opened the software, I got a flash of a disconnect window for the mouse, which then connected immediately. Note that this did not happen the third time I opened the software today. It's been a day or more having the software installed and there have been no other Bluetooth issues.
The software opens with dual panes so you have one window to select your DAP and another to select where you have your music files. In my case they're on an external hard drive. At the bottom of the window are commands, including Copy which you use to copy the music file and then paste into the DAP folder of choice. (If you use the Move command, the file(s) are removed from the source drive, not copied.)
Perhaps this software will work for you...