I tried the whole thing with a Pi 4 and the current beta version of our Buster image and it worked.
Here is my approach:
1. Installed Sqeezebox Server v8.2
2. Installed Bluetooth
3. Rebooted the System
4. Scanned for BT Devices
5. Connected Smartphone
6. Installed Bluetooth for Squeezebox Server
7. Rebooted the System
8. Connected Smartphone again
9. Started BT playback in the Squeezebox Server (Favorites –> Play Button)
10. Started playback on Smartphone
The same should work with the stretch image. Please test this again and write me if it works. If it doesn’t, we’ll take another look at the stretch image and possibly come up with a fix. Please note that Squeezelite must run with your sound card as output device (in the advanced settings of Squeezelite) for the stream to work.