If you want polyphony, buy a polyphonic synth. Ganging monophonic synths or even Eurorack isn't very efficient.
I wouldn't be afraid to get into Eurorack. But another semi-modular synth would be way less expensive than diving fulling into Eurorack. There are a lot semi-modular and even synth-voices (complete synths in Eurorack format).
I would probably go with a skiff and pick up modules to supplement the Mother 32 where you feel it is weak or to give you options with alternate filters and effects. Effects like wavefolders, distortion, ring modulation, CV modulation, attenuverters, etc. can spice up what you have.