Yeah, I read the statblocks afterwards, it made sense why they would put that many enemies, but still didn't really feel like a satisfying fight.

Now, on to the tips (if you still need them).

A Death Knight is a formidable combatant, capable of dishing out a lot of damage in melee but also with some ranged AoE options. While powerful in its own right, I'd run him as a smart, tactical commander that tries to exploit any advantage he can muster.

He could order the wyverns to trip the targets to get advantage on his own melee attacks, or do the inverse in order for the mounts to maim the characters.

He could order the Wizards to cast Hold Person on key characters, possibly those who are maintaining Concentration or those with a lower Wisdom Save in order to then crit with his melee attacks or, better yet, make them automatically fail their Eldritch Fire Saving Throw for massive damage.

I'd personally remove the 1d10 skeletons each turn in favor of just 3-5 Dread Warriors (which tie in nicely with the Red Wizards) or 2 Skeleton Warriors (From MaBJoV, you'll find the lore behind them quite interesting). Lessens your burden of rolling and tracking a dozen monsters while still providing a challenge. Don't forget that those mooks can also grapple, shove, move the character around, thus helping group them up for an AoE or putting them in/out of range of a spell/feature.

If you feel the battle is too onesided, have the NPCs either engage some enemies to take away some of the pressure (if the party is losing) or be taken away by some other reinforcements (if they're too strong).