And then I realized that all I really needed to do was slap a few resistances and immunities on one of the NPC wizard statblocks in the back of the Monster Manual and call it a day.
Using the cheap-n-cheerful templates below you can make undead archmages, bandits, cult fanatics, gladiators, etc. Need an undead knight that is less powerful than the death knight? Major Undead template + Knight stats. Undead viking? Minor Undead + Berserker.
You could also use these with regular animals, you know, for when you need a ghostly Killer Whale.
Minor Undead
Damage Resistances necrotic
Damage Immunities poison
Condition Immunities exhaustion, poisoned
Senses darkvision 60 ft.
Major Undead
Damage Resistances bludgeoning, piercing, and slashing from nonmagical weapons
Damage Immunities necrotic, poison
Condition Immunities charmed, exhaustion, frightened, paralyzed, poisoned
Senses darkvision 60 ft.
Incorporeal Undead
Damage Resistances acid, fire, lightning, thunder; bludgeoning, piercing, and slashing from nonmagical weapons
Damage Immunities cold, necrotic, poison
Condition Immunities charmed, exhaustion, frightened, grappled, paralyzed, petrified, poisoned, prone, restrained
Senses darkvision 60 ft.