Difficulty
Difficulty is another hard topic. Most games handle this by increasing values. Enemies are 2x stronger, etc. We don't need to discuss if this is a good idea. It's a bad one.
But why is difficulty a problem? Imagine every game as a puzzle you have to solve. At some point you will have a good enough solution for the puzzle that when you see the same puzzle the 50th time you will become bored. This is a real problem for grand strategy games. Because here often nations feel and play quite similar so the solution to your problems of economics, warfare, diplomacy boils down to the same solution over and over again.
In the end the player will memorize all good enough solutions and the game will be solved.
Most games introduce new gimmick mechanics for specific factions or regions. But they are missing the point It's not about new small puzzles because the overall puzzle of the game does not change. The overall game is still chess but now on the side you also need to play Tic-Tac-Toe.
These problems clearly shows in Crusader Kings 3. Here in Spain you play chess and Tic-Tac-Toe. While in Iran you play also chess but instead of Tic-Tac-Toe you play Checkers on the side.
The overall puzzle didn't change, so your solution does not need to change. At some point you will "solve" it. And yes even chess can be solved: "because I know what Chess is all about! It’s all about memorization. It’s all about pre-arrangement…".
Solution?
Games should have a way to shuffle the pieces, it should minimize the elements of the solution that can be memorized. How you would do that is an intellectual task for the reader. If the solution is way too random the player will think his result in determined by luck or just chance. The bad solution would result in the player losing control.
I can't offer a direct solution, just a different perspective on how to think about difficulty.
Think about that more. What's the point of developing a feature that is quite cool and interesting, but the player does not use it? Was the time well spent?