What should I add to my zombie deck?

without using synchro monsters, i would say pretty good job. the only thing is that zombies are pretty bad without synchros so you might want to change your deck to fit that type of build. first, take out all of the monsters that are above level 4 except maybe ryu kokki. then, take out any other monsters who dont help to swarm; which is to get out as many other monsters as you can as fast as possible. add a plaguespreader zombie, have 3 zombie master, definitely use mezuki, and use 3 pyramid turtle as well as 2 goblin zombie. as for the extra deck (the cards on the side that contains synchros and fusions) use doomkaiser dragon and revived king ha des. apart from that, throw in whatever synchros look good as well as they match the deck and are easy to get out. Hope this helps :)