I would have to run the system more to get a stronger read on it, but from what I can tell, honestly? I genuinely think that 3-4 grit is universally the sweet spot for most NPCs, even the tough ones.
Since grit is effectively "warm up"/"wasted" damage, having high grit and a high str die is genuinely painful, and a lot of the Cairn monsters by default have both if they're meant to be chewed through, something that favors Team Monster more than the players in a lot of situations.
I genuinely think that 3 grit with a d8 strength die (let alone a single point of armor) is genuinely more difficult to kill than even a 10 grit creature with a d4 hit die since the variance on a d8's rolls (not to mention the attack rolls targeting it) can result in it feeling like if you could just get ONE hit in you'd be able to bulldoze through it, whereas with the 10 grit creature it feels like wearing it down is inevitable, just that it will get its attacks in while you do.
I'm going to try the "All NPCs have 3 grit and their durability is based on armor and strength die" approach myself and see how it goes, I will let you know!