I'm attempting to set up a Necromancer as a mid-game boss for my current campaign (the necromancer is neutral-good, the party behaves in an evil fashion), but looking over the rules of necromancy it seems like even the high level template in Volo's looks like it would be an easy target for 5 level 7 PCs. Hordes of zombies and skeletons pose no real threat, especially in the face of spells like Spirit Guardians.

What tricks do you employ to make necromancers seem like more of a threat?

Cheat. 

Have the undead hoard include other creatures. Include Ogre Zombies and Minotaur Skeletons. Include Ghosts and Ghasts, Wights on Skeletal Warhorses, Wraiths, Mummy Lord, Vampire Spellcaster, and a Death Knight riding an Adult Red Dracolich.

In other words, don’t use a D&D necromancer, use a Warhammer Fantasy necromancer.

Use beefier undead stat blocks, either from official content or from stuff like Kobold Press' Tome of Beasts. KP has a ton of good stuff.

Due to the 14th level necromancer ability, you could have them controlling a tanky undead of similar power level to themselves. Be sure to choose a frontline undead with lots of HP to complement the necromancer, such as a tyrannosaurus zombie, a sword wraith commander, or even a devastating boneclaw.

I'm all for this approach, but the thing that kind of bugs me is that there's no clear path within the D&D rules for a Necromancer to create many of those monsters to have them serving as their minions - so the NPCs end up with abilities the PCs don't have access to.

Quote from bragrman >>

I'm all for this approach, but the thing that kind of bugs me is that there's no clear path within the D&D rules for a Necromancer to create many of those monsters to have them serving as their minions - so the NPCs end up with abilities the PCs don't have access to.

 That's the nature of the game. The monsters will ALWAYS have abilities that the players don't.

Quote from bragrman >>

I'm all for this approach, but the thing that kind of bugs me is that there's no clear path within the D&D rules for a Necromancer to create many of those monsters to have them serving as their minions - so the NPCs end up with abilities the PCs don't have access to.

 Yes, that’s the point.

https://warhammerfantasy.fandom.com/wiki/Vampire_Counts

Quote from bragrman >>

I'm all for this approach, but the thing that kind of bugs me is that there's no clear path within the D&D rules for a Necromancer to create many of those monsters to have them serving as their minions - so the NPCs end up with abilities the PCs don't have access to.

 That's how the game works. 

Luke never shot lightning out of his fingers.

Bilbo didn't breathe fire.

Harry Potter didn't control an army of soul eating wraiths.

James Bond never has a mansion full of bodyguards.

Harry Dresden isn't a vampire.

BOLOs can't fly - well ok, they kind of can, but not really.

The majority of the Federation Fleet cannot cloak.

Alternatively you can make a necromancer more powerful by giving him some of the OP necromancy artefacts from the DMG, specifically the Eye of Vecna, the Hand of Vecna and the Wand of Orcus. They probably wouldn't be used by a good necromancer though, but you could always homebrew your own artefacts with similar effects.

Quote from bragrman >>

I'm all for this approach, but the thing that kind of bugs me is that there's no clear path within the D&D rules for a Necromancer to create many of those monsters to have them serving as their minions - so the NPCs end up with abilities the PCs don't have access to.

True. The short answer is, "monsters don't use character rules."

In this specific case, I suggest you give the high-level necromancer more than just undead minions. Give them some apprentice necromancers, who all have their own undead. Give them some hired muscle of some kind. Give them some mundane pets (guard dogs, for example).

For interesting contrast, give the evil, gloating, nasty, despicable necromancer some polite, clean, honest, honourable hirelings. The sort of people that your gran would approve of marrying into the family, except that for some reason they serve a nasty necromancer.

Give the necromancer some mundane servants as well. Someone has to cook the food and mop the floors. I'm sure we can all agree that zombies are rubbish at household tasks. 

Evil Necromancer: Minion! This cooked breakfast is horrible! What did you put in the porridge?
Zombie: Braaaaiiiiins.

Quote from bragrman >>

I'm all for this approach, but the thing that kind of bugs me is that there's no clear path within the D&D rules for a Necromancer to create many of those monsters to have them serving as their minions - so the NPCs end up with abilities the PCs don't have access to.

Having high end minions is pretty much designed to be a npc-only ability because it causes problem when PCs have access to it, though you can always just have an 18th level caster with True Polymorph.

Hordes of skeleton archers are kinda dangerous, though, because they aren't all that easy to AoE down.

Have the stitch together a assortment of corpses to make a beefy, chunky, zombie casserole.

Quote from monkmulticlasser >>

Have the stitch together a assortment of corpses to make a beefy, chunky, zombie casserole.

 You mean kinda like the flesh golem?

Quote from IamSposta >>

Quote from monkmulticlasser >>

Have the stitch together a assortment of corpses to make a beefy, chunky, zombie casserole.

 You mean kinda like the flesh golem?

 less frankenstein-y, more more like a bulging, shambling abomination with 7 arms and 3 heads

So more like an Ettin crossed with a Five-Armed Troll.

If you ever want to introduce characters to a spellcasting NPC who mostly follows the same limitations of PCs for how their magic works, the easiest way is to imply that they're taking advantage of some unique item or power source that would be undesirable for a Player Character for one reason or another. Maybe they used some kind of unstable power source used to increase the NPC's power beyond the usual limits of magic, but which drastically reduces their lifespan or which is driving them insane. Maybe they gained a massive increase in power to control all their undead minions... but doing so required a massive number of human sacrifices... beyond what the players themselves could reasonably accomplish, even if they do lean toward Evil. 

As a DM, your NPCs and Monsters don't need to follow the same rules as players. But if you want to not stray too far from the rules established for magic and how it works, just introduce a wild card that alters things in some way. Establish that what they're facing off against IS unusual within the context of the world, and in many ways that's what makes it dangerous.