The CD that came with the device listed RTL8671B, but it's not on Realtek's website. Instead the manual points you to:
https://www.mytechkey.com/pages/driver
4.Bluetooth 5.0 Dongle
Driver Link:
https://www.dropbox.com/s/gwo9lz777i82f70/BT5.0%20Driver.zip?dl=0
Hope that helps someone.
https://github.com/peteristhegreat/drivers-backup/releases/tag/1.0
Direct link
https://github.com/peteristhegreat/drivers-backup/releases/download/1.0/BT5.0.Driver.zip
Adding visibility to @carkrueger 's comment below
If you look here: https://www.catalog.update.microsoft.com/Search.aspx?q=realtek+bluetooth+1.6.1015 You will find now: 1.6.1015.3014 from 11.01.2021
Direct download link: http://download.windowsupdate.com/c/msdownload/update/driver/drvs/2021/02/0d13721d-7aa7-47cf-887a-f53bcd228a5e_04c173bad44cec04d528d260392958dc6e5bcec3.cab
for backup purposes: h**ps://web.archive.org/web/20210216175344/http://download.windowsupdate.com/c/msdownload/update/driver/drvs/2021/02/0d13721d-7aa7-47cf-887a-f53bcd228a5e_04c173bad44cec04d528d260392958dc6e5bcec3.cab
Direct download and the popup window for 1.6.1015.3014
didn't initially work. I ended up needing to right click the link and select Save target as
.
For it to work for me on Windows 10, I unzipped the cab file into a folder and then used Device Manager to look at that folder. Worked like a charm.
You jumped the gun a little bit, 1.9.1038.3003 was released for rtl8761b a month after you asked.
It doesn't have the rtkbtmanserv from the old 1.6.1015.3020 driver but I don't know if we really need that at all. I installed older driver first and new on top of that just in case. There's probably an in-between 1.8.10x driver for RTL8761b as well but even I am not OCD enough to try to find it.
As discussed earlier, windows only supports SBC, aptx (basic) and AAC, so the choice is pretty limited. There's a "wireless sound card" like the aptx low latency dongle I mentioned if but that's really for low-latency gaming.
Soundguys have tested and measured BT codecs a few times.. https://www.soundguys.com/understanding-bluetooth-codecs-15352/
SBC is worst but depending on implementation not necessarily that bad. AAC is a crapshoot on Android phones, no idea how well Windows deals with it.