For sorting stems and seeds out of ground material there are two basic methods. Using a shop vac creates air separation that works on the basis of weight. Alternatively, gravity separation works on the basis of density. Look at seed cleaning machines and mining equipment for examples. Get a 1’x2’ Rubbermaid box, throw some ground biomass in one end, and then play with bouncing it up and down while the box is angled. You will quickly learn to gravity separate by hand. This can also change your grinding method into a better one where you don’t grind up bits of stem.