Maybe instead of having 2 stairways to get to "the top" to then be shot, make the staircase more "pyramid shaped" (or something) with a big base. In this way, zeds spawning from any direction have a straight line to get to the top (less running around to get to the stairs) and it also widens the area zeds can come up the stairs, so they dont block each other on the way up. I would think that the "top" would need to be larger than a few blocks so they dont all block each other at the top (so like the top is a platform vs a few blocks). Additionally, having 1 person "higher up" that can see down the sides of the stairs and can shoot before they reach the top might help with any weird congestion, and birds (maybe have that person more exposed to be able to shoot birds/birds attack them and bunch up?) and zeds stuck anywhere.
i feel that the more zeds you can take out "as far away as possible" the better, because the time they spend running to the kill zone takes up time for any more zeds to spawn, so the person "up top" would be 1 shot sniping as much as possible (and handling demolishers), and the person below just mowing down anything that makes it to the top.