I 5000% endorse the suggestion to look at integration. The MX2 course has a topic on integration which is an extension of earlier work. I found last year that trying to understand MX2 integration while still trying to get comfortable with basic integration needlessly complicates material.
By way if analogy, when you learned trigonometry you started with right angle triangles and SOHCAHTOA. Later, you did angles of any magnitude, and radians, and double angles, etc... Imagine trying to learn that all at once instead of learning basics then extending them once you are comfortable.
I know of several schools that are moving some integration back into year 11 (where it belongs), so that students can learn to deal with polynomials before integrating trig functions or exponentials / logs and then learning substitution. MX2 extends this to reduction formulae, integration by parts, etc, and you don't need to be trying to recall basic facts when doing that, you want those basic facts to be settled.
The Advanced course also has probability and ideas like probability distributions that will be extended in MX1 and which are ideally understood before moving on.
