I have had success playing multiple files to the onboard sound on the Pi by removing the reference to dmix (for the onboard audio only, it works fine for my USB sound card) and instead using the auto-cofigured (Raspbian Buster) sysdefault alsa device at the end of my alsa configuration chain.
I have two virtual plug interfaces, each with softvol, that ...
Section 3 at the bottom of the installation guide Troubleshooting part:
Couldn't acquire D-Bus name: org.bluealsa
It is not possible to run more than one instance of the BlueALSA
server per D-Bus interface. If one tries to run second instance, it
will fail with the "Couldn't acquire D-Bus name: org.bluealsa" error
message. This message ...
The subdevices are for multiple audio services usage that would be mixed to input/output.
There are also subdevices that are a part of an input or output
device. There must be at least 1 subdevice. In the context of output
devices, a device having multiple subdevices means the hardware can do
mixing, i.e. it can take multiple streams of PCM and mix ...