That cap is hidden and (maybe over cap?) you need quest stones because if you don't, the minus values will still be a big deal. That means mythic difficulty bonus alone isn't enough to help you get to cap.
You need mythic 3 175% exp bonus +6 quests stones.
Believe me, I was in the same boat as you until I discovered myself which works and which doesn't.
When you got 200% , with exp shrine (enshrined version), your exp turns from 200% to 500% . Then if you had exp boost, that 500% turn into a value like over 1000% (I dont remember the exact value).
It would be 200+ enshrined boost exp + 175 (and shop boost)
200+300+175 = 675% exp gain maybe. Without any boost at all, that 200% exp gain seem to be 375%, though adventure page doesn't show extra 175% exp gain bonus from m3.
M3 definitely makes a difference for the fastest exp you can get with of course 200% exp gain on gear from quest. If you try very easy mode, you won't get as much exp despite having 200% exp gain on gear because of little to no difficulty bonus. I know this from experience.
Hope this explanation helps. Main point is 175% exp gain from m3 is above cap and max exp gain on gear must be 200% (6 quest stone).
Feel free to correct me if I made mistake @Griffin012 .