There are a few ways of looking into that though.
If you just dislike threat at all, disable it on the game rules and move on, no threat for you.
If you want to play with it on, keep in mind that you will get more threat if you have a lot of land.
Generally speaking you will start getting threat when you are at a Kingdom level (or around 40 holdings I guess) but at this point it is pretty manageable, once you get very big threat will just stay there and not be very manageable at all, and this is the way it is supposed to work, and honestly, it does make some small amount of sense.
It could be improved if it worked more like Aggressive Expansion on EU4 but I'm not sure if that is doable on CK2.
That being said, the bonus provided by the councilor could be increased. I believe a sensible way of it working would be so that 100 threat goes away in 50 years with a skill 20 chancellor (it still prevents blobbing but with the added effect of not making it completely unmanageable).
Or just ignore it and quickly siege the wargoal to end the war sometimes it gets annoying when you're attacked on multiple sides and the wargoal alone doesn't give 100%, but generally speaking it works.