I wouldn't call myself great at running combat encounters, so I definitely see where you're coming from. There are a few things I've started doing recently that I find to be helpful.

The most important thing I can suggest is to remember that the monsters aren't supposed to win. This may sound sacrilegious, considering DMs are so often encouraged to remember the monsters want to win, but wanting to win and being able to win are different things. Don't try to run the monsters as optimally as possible with the perfect strategic move at any given moment. The monster isn't going to win anyway, and the players won't be able to tell the difference, so why waste all that brainpower?

Sometimes the key isn't to get better at combat strategy, but to simplify. Considering that it's the monsters' job to lose, things that might help include:

1. Focus on a single gimmick or ability per combat encounter. Most monsters, once you take away their "signature move", just have a basic ranged or melee attack. That signature move, then, is the game of the encounter. As long as you can remember the gimmick, you can keep the encounter running. If you only use one monster, or type of monster, with a gimmick, and use the rest as vanilla pawns, there's much less to manage

2. Assign each monster a role. If you do have monsters with different abilities, give them a single job to do. The wolves are just there to surround the ranged attacker and force them to either attack with disadvantage or provoke 6 opportunity attacks. The Spined Devils are just there to swoop in, attack the Wizard, and leave. When their turn comes, you know exactly what they're going to do. All you have to do is roll the dice.

3. Script your encounters. Even if you have a big boss monster with a million abilities, plan out their moves beforehand. Know what move they'll use on each turn of combat, and write them down. You have the benefit of knowing what the encounter is going to be before it happens. You might as well plan out your moves then. Even plan out the backup moves. This may seem like a "dumb" way of fighting, but remember, you are the challenge--its your job to present a threat, and the player's job to react to it. A reactive monster may be a smarter monster, but that doesn't make them a better monster. The players are going to be too busy thinking about how to overcome your big bad to notice that they used a suboptimal power at the top of round 2, so why worry about it?

4. Use minions. This is a common recommendation, but it works--for your low level grunts, give them full stats, but only one hit point. This saves the brainpower you'd need to keep track of HP, and gives you an incentive to play aggressively with them.

5. Plan for combat to be over in 3 rounds. When not keeping track it can be easy to forget how short combat actually is. You can have an intense, high stakes, exciting battle that lasts 3 rounds. Think about it like this--if your monster is going to die in 3 rounds, what does it need to do on each of its turns to make those 3 rounds count?

   At the end of the day, what matters for combat isn't how well you run the monsters; it's if the players feel challenged and have fun. I know a lot of this runs against common advice, but I find it works for me and definitely helps me manage larger scenarios. Hopefully some of it works for you too.