I think people say it's too hard because they they get caught up in the trees while looking for a forest.
For starters, you know how you run monsters, you don't have to guess at what strategy you will be using for them. So that's half of the combat solved already!
Secondly, you don't have to account for every single player action, just the most likely ones. After playing for 10 levels, you should know that the Cleric is casting Bless, the Sorcerer tosses out Fireballs, the Barbarian runs up to chop the biggest threat with their axe, and the Wizard is going to be doing some kind of crowd control.
And the hardest part of that calculation, the crowd control, is way easier to account for than people think it is. All you have to do, is plug all the monsters into the action economy and subtract some actions. You just figure out how much hurt they do per action, and then guestimate how long they will be out of actions for.
Like if Web is the control spell of choice, you take 1 action away from everything likely to be caught in the Web. Because Web is designed to be broken or burned, and you can just save against it if you are lucky.
On the other hand, if it's something like Force Cage, well that target is just sitting out for the entire fight. But still easy to calculate because they don't start damage till everything else is gone.