May I suggest an integration of OPEN STAGE CONTROL (I know Open Sound Control is already embedded…) ? This software is highly customisable and allows the creation of a MIDI controller directly on the device… which means the user would’nt need a second device to control the different engines, this would also solve the favourite controls panel implementation or OSC setup, meaning the server AND a client could be running simultaneously on Zynthian…
https://openstagecontrol.ammd.net/docs/getting-started/introduction/