As Dexed’s Monophonic function cannot be manipulated other than manually in the plugin’s GUI, I use 2 other solutions that suit me for monophonic playing:
- as a guitarist, I use Midi Guitar by Jam Origin and simply tick the monophonic function:

- with a midi keyboard, I use the MidiPolyphony plugin by PizMidi inserted before Dexed

I set note priority to “steal oldest” and all MIDI channels to the same channel (your choice) and bypass for polyphonic mode.
