You want to use spent grains as critter feed? Is that correct? For turning spent grains into protein for human consumption, there's no need for any precomposting or black soldier flies. First off, you and your animals can eat it straight away. I don't have animals and it tastes bland (also has sharp chaff bits that make it unpleasant unless ground) so I usually just throw my spent grains in the back yard and it's gone in a couple days from the birds flocking over it and squirrels grabbing it up. I was watching a squirrel dig up my recent homebrew grains from the snow just this morning. I can't say from experience but I have heard, and would bet, that chickens, pigs, and cows would be very happy for it. I hope I understood you right, but if not, the worms, flies, and fungus would definitely munch on it too!
In case you're not familiar with brewing, here's a quick overview. Alcohol is made from sugars, so the brewing process starts by converting starchy grains into sugary grains. For beer it's mostly barley, with additions of wheat, rye, and oats, and possible adjuncts of rice and corn. First the grain is malted, which basically means it's moistened, sprouted, and then dried. It can then be safely stored for long periods until ready for brewing. I know I've read that sprouts are supposed to be more nutritious than raw seed, so I like to think that about malted barley too
Anyway, after that the malted grains are crushed and then steeped in hot water. The specific water temperature activates digestive enzymes in the malted grain that convert starch into sugar. When all the water is drained off, it takes most of the sugar with it and that is what goes on to become beer. What's left are your spent grains: fiber, proteins, various minerals, starches that didn't convert, and sugars that didn't drain off.
Now, in the off-chance that your spent brewery grains have been mixed with other brewery waste, I can't really comment but would caution against consumption. Here's more brew process for ya. After the sugar water is run off, it's boiled with hops, which act as a natural preservative. Then it's cooled and yeast (fungus) is added. The yeast replicate rapidly in the initially aerobic solution, then they start anaerobic respiration (fermentation) of alcohol and co2, which turns the sugary liquid into beer. The waste from THIS part of the process is a small amount of grain particulate that settled out of solution, maybe a few bits of hop, and a whole bunch of dead/dormant yeast cells, as well as trace alcohol, hundreds of various yeasty byproducts, and after exposure to air there's definitely a bit of wild microorganism activity, like lactobacillus or acetobactor. I occasionally dump that waste on my compost, but I don't like to make a habit of it and I definitely don't eat it for several reasons. 1. It's in a highly anaerobic state that would require significant energy on my part to prevent clogging up my nice aerobic compost I work so hard for. 2. It tastes terrible to humans so I have a hard time imagining animals or even worms would feel that differently 3. Yeast is unhealthy as a dominant feature of my gut flora, so it's inadvisable to inundate myself that way. Instead, most of that goes down the drain or spread thin beneath a hedgerow.
I hope that helps.