I see two broad options, one of which is a bit of a frame-change.
First, the frame change: most undead - especially low-level undead like zombies and skeletons - were mindless in previous editions of D&D and aren't terribly bright in 5E. It shouldn't be too hard for some passably persuasive denizens to convince the undead hordes to follow their commands without magic. Further, zombies and skeletons are frequently portrayed as basically milling about doing nothing in particular if there's nothing better to do; it shouldn't be too hard for the denizens to have corralled the undead into holding areas for later use (it's not like they need food or exercise, after all).
Option 1b, I guess, would be that there's no particular reason for the undead creatures to stop doing whatever it was that they were doing when they're no longer controlled, especially if what they were doing is a mindless, repetitive task ("keep turning this wheel" or "walk along this round pathway (ie., patrol)").
If that's not quite sufficient, the denizens could be keeping a handful of key undead creatures under their control, and trusting that the rest of them will just kinda follow along.
PCs could, in principle, do the same thing: control or convince (or just lure) the undead to where you need them to be, then let them hang out there 'til they're needed. If necessary, keep a small number under control for specific purposes.
The second option would be to adjust (slightly) the Hallow spell, and "Un-Hallow" the area (which may indicate that a more powerful creature is behind this, and the adventure is prelude). As written, Hallow appears designed for Good PCs to use; I don't see why its mirror shouldn't exist, which would allow for necromancers to exert some control over more undead than normal (possibly only just "some" control, though: perhaps it's just enough that the undead see the other denizens as "friendly" as opposed to "lunch").
Again, in principle a PC could do this, if they could find a deity willing to allow it.
I sometimes make characters and monsters to use their Reaction (if they still have it) to perform things like releasing enemies or fleeing in terror. I actually got the idea from an existing spell in 5th edition, Dissonant Whispers (pg. 234 PHB):
The target must make a Wisdom saving throw. On a failed save, it takes
3d6 psychic damage and must immediately use its reaction, if
available, to move as far as its speed allows away from you.
Normally you're not able to use your reaction to move, but this spell shows that it's not impossible to be forced to do unusual things with your reaction via magic.
In the case of turn undead, I would possibly have the skeletons which have already acted use their Reaction to move and the rest move normally on their turn. It can be a tricky balance.
You have to be careful and decide ahead of time what a creature does with its actual turn when it comes round. Otherwise, you could make spells more powerful than you intend by having creatures run twice as far away or less powerful by creatures still getting an action on their next turn for example.
Best Answer
1.4 or 6 waves depending on how you view the battle
The DMG says that a hard encounter for 4 8th level characters should be worth 5600 XP. A single wave of 20 zombies is worth 1000 XP, but gets a x4 multiplier for a total of 4000 XP (DMG 82). Thus, by the book, 1.4 waves is enough for a hard encounter.
It's also worth noting that DMG 83 suggests that you treat each wave as an individual encounter:
If you do this, the total adventuring day XP budget for your party will be 24,000 XP, which means that your party should face about 6 waves for the entire day.
You'll need way more than that
In my experience, the DMG's encounter guidelines work best when the PCs are fighting a group of similar size--4 PCs vs. 4 monsters, for example. The further you get from a 1:1 ratio, the more likely that specific traits will make a huge difference. For example, 1.4 waves will be easy for this encounter--a single fireball will probably kill the first wave, and a turn undead would mop up the second. Your wizard can cast up to 5 fireballs (and sculpt spells), so that's probably the minimum number of waves you need before the encounter becomes anything but trivial.
Therefore, the easiest way to make a challenging encounter is to just keep throwing waves at them until the PCs begin to run out of resources. Even if you do set a hard limit on the number of waves, your players are never going to know if you planned for 2, 5, or 50 waves anyway. However, you will know if your evoker runs out of fireballs or the party starts to get overwhelmed, so you can just choose not to send in another wave after that. That's probably the best way to simulate an exhausting battle of attrition, especially if you're not going to use things like tactics or terrain.