Thanks a lot for the advice, it's really helpful. Modifying monsters in this edition seems to be a bit more fiddly than it was in 3e.
Good point. I should have specified that a class's "most iconic feature" is an entirely subjective matter; whatever you think is the most significant feature from that class is the feature you should put on it.I say give them what abilities you want to deal with. Since you're going to factor them into the CR anyway, give them what you want. Like I wouldn't give Combat Superiority over second wind, actions urge, and fighting style; combat superiority is a subclass ability, and is no more core to the fighter is than eldritch knight spells are. But I have given combat superiority to NPC warriors (The Bladebearer Hobgoblins from Red Hand of Doom for instance).
Also a good point. Combat Superiority, or other abilities with a specific number of uses and/or a range of effects to choose from, are definitely worth simplifying. For instance, when I make a monk NPC, instead of tracking Ki I just let them make Flurry-style attacks as a bonus action, and don't worry about tracking Ki or using the other Ki abilities.But, you'll see many abilities in the MM that "almost" mimic some of these things. "Parry", on many of the NPCs, kind of mimics the battle master's parry ... more specifically the defensive duelist feat. Those types of abilities may be less fiddly than giving an NPC actual Superiority (or Sorcerer) dice.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.