Seems good Tototun. u should be able to do what you want. 4 voice poly + drums and fx.
couple of things u might want to know: you have an output module which makes me think you want that as a final output but you might want to add a mixer so you can have more sources going in there .
also as for midi be aware that this exists too: https://www.tindie.com/products/hotchk155/cvocd-a-super-flexible-midi-to-cv-box/
so you can have poly midi AND trigger a lot of extra stuff for less money than the yarns.
theres about a thousand ways to design a case that does what you want so i wont give you any advise there its much more fun choosing your own modules i think :D
cheers S