Patchbox OS upgrade to Buster is planned to happen in the near future, we’ll keep an eye on the bluetoothctl version within the new image, so it’s >=5.48.
As for fluidsynth and jack - it should not be starting a new server, but rather it should connect to the one running in the background. This should be achieved automatically by having the right environment variables defined, they are pre-defined by default in /etc/environment, in particular the JACK_PROMISCUOUS_SERVER=jack. It also depends on how the ‘fluidsynth’ binary itseIf was built - if it’s using older Jack development libraries, it might not support this functionality. Where from did you get the ‘fluidsynth’ installed?
To get MIDI connections automatically, Patchbox OS does have our own solution to this problem using amidiauto, it should automatically connect all hardware and software MIDI ports together, but you can also explicitly define the connections in a configuration file.