Good idea. Sounds like things might already be starting to chug a bit...I don't want to slow down combat . . .
I have five 7th level players: vengeance paladin, lore bard, four elements monk, beastmaster ranger, arcane trickster thief.
+1 You might reduce the zombies from being NPCs a step further, and turn each one into a miss-consequence. As in: on a hit, you destroy the zombie. On a miss, a zombie gets to attack you. The zombies don't get turns, just these reactions. Increase the number of reactions as the horde, IF the horde surrounds the PCs. And keep the Zombie Count in plain view - lest the PCs lose hope.There is an interesting mechanic from older editions of D&D in which you could treat monsters as minions. Essentially, they retained all of their normal stats except for their hit points, which were reduced to 1. This can create the feeling of the adventurers cleaving through massive hordes of enemies and dealing a lot of damage. However, because they have only 1 hit point, you may need to increase the number of zombies in the encounter.
One (of the only) thing(s) the World War Z movie did right was the way it treated zombie hordes not as swarms of individual monsters but as forces of nature. There's the iconic scene of zombies swarming through the city of Jerusalem like a tidal wave.
Not sure if what you're doing exactly qualifies, but I would imagine beyond a certain size a zombie horde ceases to be a combat challenge and becomes instead an extreme environmental hazard.
*Edit: Ninja'd
But we appreciate your attention to detail
Edit: Searching for the swarm trait I found this relevant article: http://slyflourish.com/high_level_swarms.html