I think they're foundationally different types of games, though. EU4, in a really oversimplified way, is kind of a game about numbers going up. You're in a constant arms race of progress to have the biggest numbers, and you want your numbers to keep going up so you can have bigger numbers than the guy next to you. It's much more standard grand strategy in that way, and having a difficulty that makes those numbers harder to push up works well for making that game more interesting. I think, for a game about characters and relationships though, simply changing those numbers means less.
To make a weird analogy: EU4 is like a marble race, and CK3 is like a pinball machine. EU4 is a pretty linear game, in that you are generally trying to get stronger, maybe in taking more territory or maybe just in the quality of your wealth and power, but once you suffer a few catastrophic defeats, the run is kind of over, the same way that once a marble falls behind in a linear race, it's probably over for it. The trajectory of a Crusader Kings game, though, is more like a pinball machine: it's the most fun when it's bouncing all over the place. It's not fun to get the ball in the hole as quickly as possible, but to juggle the ball for as long as you can, watching how it bounces around and seeing how long you can last, but knowing that, inevitably, you're going to lose.
If you just make the paddles on a pinball machine weaker to make it harder to juggle the ball, it doesn't make the game better, it just makes it more annoying.
Simply making characters more efficient at the things they're doing isn't what makes CK3 interesting, in my opinion. What makes it interesting is that they are doing things, giving you weird obstacles to bounce off of and take your game in new and exciting ways, and I don't think that's something you can just throw numbers at. I think that's something a lot more difficult and time consuming to adjust because it requires making the game bounce the player around in interesting yet non-rage-quit-inducing ways (poor Harm events, you were a good idea, I'll always love you).
In fairness, I agree that there are definitely some numbers that could be tweaked to make the game both more interesting and more difficult, but I don't think simply making the game more difficult in terms of the efficiency of other characters (or the inefficiency of your own character) works as well in a RPG-esque strategy game like CK3 as it does in EU4 or most other grand strategy games.
PS. I apologize if I've misrepresented or misunderstood EU4, that's just how the game feels to me, but I'm very bad at it and have only played a few hundred hours (what a weird thing to say—only in Paradox games, amirite?).
That's the issue, in terms of combat CK3 is simpler, perhaps the simplest paradox game ever made, but it has the exact same dynamic.
The only difference is that in EU4 the AI actually realizes there's a race, competes with you, and often stays years ahead of the military tech worldwide, while in CK3 the AI doesn't realize there's the exact same dynamic, watches as the player gets +500% damage on his MAA while every AI in the planet remains with the exact same armies they started the game with.
Hence the "it's all naked corvettes with red lasers" analogy from Stellaris.
If you were playing against a proper EU4 AI in Stellaris, by the time you researched cruisers some enemies would have battleships, everyone else would have cruisers, and you'd have epic battles against giant fleets just like your own.
If you were using the CK3 AI in Stellaris, by the time you reached cruisers the AI would still have the 20 naked corvettes without any technology using basic weapons they started the game with.
There is no number modifier you can slap on it and make the game functional, the AI isn't bad, it's turned off.
While you were starting your early conquests from a single county: The AI did nothing
While you were looking for allies or partners with good stats for your family: The AI did nothing
While you were building up your domain & economy: The AI did nothing
While you were working through your vassal contracts to get better deals, lowering levies (useless) in return: The AI did nothing
While you were conquering your first kingdom: The AI did nothing
While you became the cultural head and started researching technologies that actually helped you: The AI did nothing
While your culture evolved and you could add new traditions: The AI did nothing
While you were gathering artifacts for your court & family: The AI did nothing
While you were landing your own family and getting more thrones for renown/stability: The AI did nothing
While you were done with economic buildings in your domain and started building a few military ones to station your MAA: The AI did nothing
etc....
Is it any surprise that after 20 hours of gameplay, in a single game, every encounter ends up with an instant stackwipe? While you had 20 hoursof content/actions taken, the AI literally sat there and drooled.
The only time in which the AI can be slightly challenging is in your very first encounter, in your very first war, specially when you don't start it and a bigger foe attacks you, anything after that experience goes downhill as the AI shows no reaction and never attempts to take a single action, in intrigue, in economy, diplomacy or warfare to keep up with a player that doesn't just stand there going afk at speed 5 clicking events.