Before you tackle Idyllic Poem, which is quite difficult, you might go into Poem of the Little Stars, which will give you phrasing and harmonics practice, and the Preludes Intimes, if you haven’t done them already, which are excellent mood pieces.
It will also help to do exercises on double thirds and other intervals, like he uses in Idyllic Poem, and the descending fifths and sixths in one hand. There is something about coordination in that passage that makes it so hard, even though it is in some ways simple.