KISS is a standard rule of design. Often though we forget about it when designing our encounters. It's a natural thing to happen. You want a fight to be cool. You want the big NPCs to be cool. And being cool means being able to do special stuff. I mean, all the 'cool' stuff your PCs can do comes from abilities they can do that trump or otherwise modify the normal rules of combat.
The thing is, when you are a player you only have one thing to worry about: your character sheet. When you are the GM you have a ton of things to worry about. Everything from initiative order, to every character - yes even the PCs - in the encounter, to all the special abilities, to health levels, to the environment. Not to mention things like rules, rule disputes, and helping players figure out what they can and can't do.
This is why things like enemy spell casters can be a huge pain in the ass to run. Unlike the players you can't spend the round reading and deliberating what spells to use. You have to be resolving every action and deciding for every NPC.
Because of that, try to keep your encounters simple. And by that I don't mean every encounter has to be the same. But don't involve more mechanics than you can easily keep track of. Which is why I prefer Matt Colville's "Villain Actions" over the standard 5e Legendary Actions. Legendary Actions are complex. If done with a solo monster it works, but when the solo monster has allies it is more things to juggle and gets weird. A Villain Action on the other hand is simple: round 1, this happens. Round 2, this other thing happens. Done.
What if you can't keep the encounter simple for whatever reason? Well, that happens. Try to avoid it, but you can't always. In those cases, go in with a plan. Write the plan down too. Give yourself a cheat sheet to go off of. This is how "The Monster's Know What They're Doing" basically suggests with his strategy options for monsters. Round 1, this monster does X if possible. Round 2 it does goes to its next ability.
No plan survives contact with the enemy, but going in with a plan gives you something to pivot from. Going in without a plan aside from "the bad guys ambush the PCs" gives you nothing, and leaves you having to figure out complex arrangements of abilities from 0.
Give yourself the safety net. Take the time to make yourself a plan and a cheat sheet. And whenever possible, don't make encounters any more complex than they need to be in order to work.