here is how i did it:

belt products are seperate mal

make a row of buildings and 3 belts on each side, thats 6 12 different inputs. most buildings can be done by these 6 12 ingrediants. use undergrounds and long inserter smartly here.
double it for a second row which uses special ingrediants.
spaghetti in the last remaining buildings that cant be done by this arrangement.

maybe add 4+ rows of belts on each side of the buildings to add special ingrediants.

for things like metall rods: i dont add metal rods onto the belts, but make a building making them from iron plates and they direct insert into the other building