Spiral galaxies exhibit a dense cluster of stars in the middle, which is what I was seeking to implement.
I thinned out the cluster a lot from the original setup I was testing, so that the smaller quadrants would have less in there and more in the arms, while the larger quadrants would still have a dense cluster. There are only 13 fixed system positions in the center; the rest are generated by the spirals themselves. Here is an example of the Small quadrant:
