I've used https://www.bluetoothgoodies.com/a2dp/ for a while on a couple machines. License costs a couple bucks and is tied to a machine but it's very cheap. I haven't found any useful open source options unfortunately.
Ah sorry, I missed a specific there. I imagine that guy is trying to add it, but yeah not in there yet.
From what I've seen, you might want to go pure hardware, with a portable dac or similar from someone in the HWA like Fiio. Many can be plugged directly into a Windows machine and perform the rendering and Bluetooth transmission directly, so you don't have to mess with the crappy nonsense in the Windows Bluetooth stack.
I've used some Fiio devices regularly over the years and been pretty happy with them, but your needs might vary. Currently I have a btr15.