Apple - Prevent auto-pairing for certain devices (Bluetooth)
I had the same problem and FINALLY found a workable solution. Run this in your terminal. After this you just need to manually connect to your bluetooth audio system.
sudo defaults write /Library/Preferences/com.apple.Bluetooth.plist DontPageAudioDevices 1
Adding MAC address of a device to IngoredDevices disables auto connect of Bluetooth. To do so follow these steps:
find MAC address in DeviceCache with following command
sudo defaults read /Library/Preferences/com.apple.Bluetooth.plist DeviceCache
add MAC address to IgnoredDevices
sudo defaults write /Library/Preferences/com.apple.Bluetooth.plist IgnoredDevices -array-add '<MAC ADDRESS>'
print contents of the file, you should see there MAC address which has been added in step 3
sudo defaults read /Library/Preferences/com.apple.Bluetooth.plist IgnoredDevices
restart your Mac
This should work on Mojave and Catalina.
This is a bit of a long shot, but at home go into System Preferences > Sound > Output and manually select the Default option (Usually Speakers).
This means that OS X will re-prioritise so that when the Belkin F8Z492-P Bluetooth Audio Adapter becomes available, you'd rather use the default option.
Then, when you want to use it, select it via the AirPlay menu (if possible) because that doesn't affect OS X's priorities. If it is not available via the AirPlay menu then you will just have to manually switch forward and back with the System Preferences app, unless you feel like diving into some AppleScripting.