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:
For such encounters, treat each discrete part or wave as a separate encounter for the purpose of determining its difficulty.
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.
When it comes to DM rulings, they are ultimately always correct by the rules as intended: the intention is that, as the DM, you have the final say in these matters. So that one's easy.
It's not correct by RAW, as the Mage Hand spell does not state anywhere that it causes Disadvantage, nor do Ogres list anything about having Disadvantage because they get distracted by magic.
That leaves us with "Is this reasonable?", which is the most interesting one of the three, I think.
If we check the spells section in the PHB, there is actually one cantrip that applies Disadvantage to attack rolls. It's Vicious Mockery, which is a Bard spell. However, using it requires an Action (as it's a single target spell), only applies Disadvantage to a single attack roll, and allows the target a saving throw to avoid the effect entirely. The only thing Vicious Mockery does, that your ruling does not, is 1d4 Psychic damage. Which, ultimately, is pretty inconsequential. So granting Disadvantage to all attacks for the entire 1 minute duration is very powerful for a Cantrip.
If we look further, the only 1st level Wizard spell (I found, anyway) that can grant Disadvantage on attack rolls is Fog Cloud. But that affects everyone, including your allies. So if your idea for Mage Hand was that it would distract all enemies, that makes this Cantrip more powerful than a 1st level spell.
So I'd say you made Mage Hand into quite a powerful spell by allowing it to apply Disadvantage to all attacks by enemies for a minute, without a save. (Not that it's a big problem; but I'd be careful allowing it to do this all the time)
Fortunately, it's easily explained in the fiction. Ogres are commonly known for being about as clever as a sack of bricks, so saying that the Ogre got confused by the mage hand makes total sense in the situation. But I wouldn't turn it into a regular house rule.
(If you want a more balanced version of "distract enemies with mage hand" then I'd say using your Action to target one opponent, who gets a saving throw, and has Disadvantage on their next attack if they fail. It would be balanced; that nets you a weaker version of Vicious Mockery, which is fair considering the rest of the utility of Mage Hand.)
Best Answer
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):
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.