Here's my take on combo zombies, original deck I saw listed below. I currently don't have the fetch lands for it, so I changed the manabase a little, and I changed a couple of the creatures numbers in the main. Other than that my sideboard will be similar and slightly tuned to my preferences.
Deck is based heavily on this list: https://www.mtggoldfish.com/deck/5332039#paper
Edit 1
I want to add that the manabase is not optimal because I don't own marsh flats or cavern of souls. Once I get my hands on a play set though this list will be edited to reflect it. I'm simply working with what I own.
Edit 2
Swapping in 4 marsh flats for the 4 snarls. 1 more Castle, 1 vault of the archangel and 1 swamp for the 3 plains. Main boarding 3 tidehollow sculler since there are multiple cards that can remove him to capitalize on permanent hand removal, losing 3 headless rider. Eventually Cavern of Souls will replace isolated chapel once I can get a playset.
Edit 3
I think this list will be the one. Will play it a bit and update as needed.
Main
-4 Isolated chapels, +4 cavern of souls.
-1 Undead Augur, -1 Swamp, +2 Headless Rider
Side
-2 Nevermore, -1 Tormod's Crypt, -1 Leyline of the Void
+2 Surgical Extraction, +1 Nihil Spellbomb, +1 Pithing Needle
Edit 4
Modified the sideboard once more.
+1 Leyline of the Void, +1 Solemnity, -2 Surgical Extraction