What are your thoughts on cards like
Corpse Connoisseur and
Stitcher's Supplier? You have quite a few effects that benefit from cards being in the grave, why not turn them on earlier and give you more card selection while you're at it? You could also run
Buried Alive, but that's neither an instant nor a zombie so I could see why you're not already running it. Looks like a pretty fun deck overall,
The Scarab God is a pretty disgusting zombie commander imo
EDIT: Also
Coat of Arms is always a solid option for tribal token decks like this.