Good luck with your project.
This can be very interesting with the amount of details you have already put in your map of the British Isles.
However, I think with the details you go for in this mod it makes sense to keep the focus on the British Isles.
This is because I think the rest of Europe would be a huge task on this scale.
Imagine the number of castles that you should place on this scale in France alone, that country really had many.
I am telling you this because I am currently planning to make a mod with similar details to yours, but in a different part of Western Europe and in a different time frame.
So I looked at the amount of work it takes to do this.
It's already a lot of places to look up if you want to make the mod historically accurate on that scale.
Of course, it's not my decision to tell you what to do with the mod, I just recommend picking up small parts of your mod one at a time and using those results as motivation not to overwhelm yourself.