Which tells me loud and clear that the PCs need some serious toning down across the board in order to fit in better with their settings.
And that, sadly, won't happen: 5e is designed to let the players wade through the vast majority of their opposition, more like a scaled-back supers game than anything the least bit gritty.
It's more...
5e was designed on vibes after 4e was designed with a purpose and people reject it because it didn't have the correct vibes.
To have NPCs that can use PC logic you have to design your NPC system and PC system together.
Think of it like 4e a monster roles. If we say that PCs are strikers: they have high damage, high accuracy, low HP, and have a way to prevent themselves from being killed (mobility, High AC, defensive spells) but die if cornered or jumped.
Then we graph a typical type of monster that fits that role goblenoids and elves.
Goblin minion, Drow minion, hobgoblin minion, bugbear minion,, goblin Warrior, Drow warrior, hobgoblin Warrior, bugbear warrior, goblin leader elf leader, hobgoblin leader, bugbear leader.
Plot out their stats and put PC offense and defense near those benchmarks, then it could be done.