well for 200 dollars you could have a dedicated control surface like this:
but that's only 9x1 sliders and 9x3 knobs assignable via midi (not including the buttons). I mean by my count you have (between the MPD and MPK) 14 sliders, 16 knobs.
I mean I guess it could get kind of annoying trying to figure out which slider/knob corresponds to which channel, however, you have to ask yourself is it worth the money to you? Its functionality you technically already have, albeit atm it might not be as convenient as having a dedicated control surface.
If I were you I would just get a legit mixer (RCA/1/4" ins) with USB and MIDI I/O and just use it as a control surface for now, so that if you ever get into buying rack-mount equipment or XLR mics or whatever you can easily turn your MIDI control surface into a method of recording sounds from synths/whatever
that's my ten cents