Designing a polyphonic synth
Looking at how Roland etc did it:


CPU decodes MIDI / scans keyboard matrix / computes envs and LFOs / etc
Single DAC (CPU writes the CV for every parameter one by one in a loop)
4051 8:1 muxes connect the DAC to the S/Hs one by one (controlled by CPU)
Each S/H is a capacitor (to store CV when not connected to DAC) and an OP-AMP


Only polyphonic analog synth I'm aware of that doesn't do this is CS-80.