How can I have low-level 5e necromancer NPCs controlling many, many undead in this converted adventure?

I require as close to a RAW answer as possible for ~Level 5 Wizard(s) to control ~100 skeletons and zombies.

Context: I’m adapting a few classics for older editions to D&D 5e, and running into issues for which I need RAW solutions. I could just hand wave it away but my own DM does that sort of thing and I am very much a proponent of what I call the Goose and Gander argument for players and NPCs: essentially, (with a few exceptions) if the bad guy can do it then so can the players, given enough time and resources. So handwaving the NPCs’ abilities doesn’t work for my campaign.

The AD&D 2e adventure Return to the Keep on the Borderlands has a Necromancer and several large groups of skeletons and zombies, but there is no possible way that the denizens of the temple could maintain control of so many for what they are utilized.

All told there seems to be ~100 skeletons and zombies that are described as being controlled, i.e. they have tasks they are performing when encountered. The module only describes a single caster that would be capable of casting Animate Dead and it is a 5th Level Wizard (Necromancer) there are a handful of lvl 3 Clerics in there as well. This technically would not have worked even in 2nd edition RAW since Animate Dead was 5th level for Wizards. I would however like to have at least a modicum of a better explanation than “Well, that’s what was written in the module.”

Therefore, is there something out of all the books that I am missing that could justify a small, essentially low level temple having so many controlled undead?

Immediate thoughts would be replacing some of them with constructs. Given the nature of the temple Scarecrows are the obvious choice and would be controlled indefinitely. I did see some ideas about converting the Bone Golem from previous editions — the problem with that is it changes the focus from Necromancy to Conjuration (as Scarecrows are bound spirits), which is not really desirable nor as interesting a threat to the good NPC side.

I also thought of scrolls but they are limited and would need to be replenished somehow from a higher level wizard that can make them. This option would require many scrolls, possibly dozens, per day if the existing casters capable of scribing them were to be the ones creating them.

I don’t see a way of doing this without a custom magic item, something akin to the 3.0 whistle from Sunless Citadel.