>>718173486
Read the entire content of the belt with a decider combinator, set it up to read Each and output a single signal of Each when X > N° and have the Inserters and enable when X is recieved, if you want to use a splitter just read the bit before merging but you should desing your spaceships to use just one side of the belt for diefferent stuff.
Asteroids go to the right processed materials go to the left and so on