Is it possible to have the hold options for Stems and Sequencer operate as a toggle? For example, press key 1 to activate hold on track one and then press again to deactivate hold and stop playing? At the moment, with Hold enabled, pressing key 1 enables the track and I can’t see a way of deactivating hold without deactivating it for all tracks at once.
At the moment all keys are transmitting midi data too which are necessary for triggering the sequencers while in keyboard mode, so it will not be possible to add this extra function to these keys. However, you can map the Hold button(s) to your controller via midi mapping.