shuttleMidi

I use it in conjunction with Reaper (I use this theme), bx_solo (free) and ReaLearn (free Reaper extension) as an audio monitor controller 'deluxe': volume, solo, mono, dim, headphones, ... The transport keys send MIDI CC (to control a sequencer or whatever reacts to MIDI) or can control the Music.app on MacOS. You need one IAC port to do all that. One more thing!: you can display the state on a connected Mackie Control (Behringer X-Touch in my case)
You can download it on github: github.com/awitez/shuttleMidi