Surprised no-one mentioned https://syncthing.net/ yet. That would seem to be the main competitor for such an app. Still, perhaps there's a market for something a little more user friendly and streamlined. Nice work!
To be fair, the developers of Syncthing have been explicit that Syncthing is not intended to be a file transfer application, and that you should avoid doing so. Syncthing is designed to keep two folders on separate devices in sync at all times.
That said, I have used it at times to "transfer" a single file that I copy paste elsewhere and then delete from the sync folder. Sorry Syncthing devs..
I have my Android and laptop running Syncthing with the Android end read write so when I move the files out of the receiving directory on the laptop it is automatically deleted on the Android device. No need for separate copy and delet, just move.
That said, I have used it at times to "transfer" a single file that I copy paste elsewhere and then delete from the sync folder. Sorry Syncthing devs..