Get a paraphonic / polyphonic synth and use Eurorack to create timbral variation of a harmony. Eurorack is not good at creating complex harmonies.
-- Wavescape
No disagreement here. Sure, you COULD build a polyphonic modular, but the resulting instrument would be QUITE huge, physically, and the patching complexity would border on total insanity. The ONLY setup I can think of like that would've been Toto's massive early-mid 1980s Polyfusion rig, which also used a Roland MC-8 clocked by a central timing device (my bet's on Garfield's MasterBeat being the device responsible) for control. And sure, Eurorack IS smaller...but you'll still wind up creating something huge and maddening to patch even in that format. You're far better off trying to set your build up as a multichannel set of potential mono/duophonic patches, then slotting those in and out via your main output mixer. Then if you want chords...there's loads of options with prebuilt polysynths.
As for 30-40 modules being a "large number", consider this: I learned modular (1980/81) on what I still say is the best teaching synth, the ARP 2600. And even though that looks to be just a big prepatched rig, the fact is that it contains 25 or so discrete "modules" within it, and you can break the prepatching just by plugging in patchcords where you see that little circled "interrupt" icon. So 30 - 40 is really kinda typical for module counts in a basic Eurorack system.