[...]I thought it would be a fantastic idea to do on my own and it turned out to be far more challenging than I had originally thought[...]
The problem with ranking the difficulties of the works of any composer lies in the choice of criteria for difficulty. What might be a challenging difficulty for one person may be less of a challenge for another, and that may be where you and many others find it challenging to rank each work of any composer against that composer's other works. Even comparative lists compiled by "professionals" are open to debate. We have seen several debates here criticizing Henle's graded system against other systems. It's all related to what is considered difficult and by whom.
Some pieces are obviously more difficult than others. Rachmaninoff's Elegie from Op. 3 is considerably easier than the Prelude, Op. 23, No. 9, for example.
Speaking of the Preludes: Taking just two consecutive examples from Rachmaninoff's Op. 23;
- One person may find Op. 23, No. 4 extremely difficult because of the voicing issues (internal sustained melody in the lower voice in the right hand against a triplet accompaniment in the upper part of the right hand). S/he would rank that at X difficulty
- Another person may find the Op. 23, No 5 more difficult because of the four-note chord jumps starting at measure 17. S/he would rank that at X difficulty.
Which is more difficult? Or are they ranked as equally difficult, but just involving different difficulties? Doesn't it depend on the individual skills of the player in question.
Therefore, trying to rank all the compositions of one composer can be an exercise in futility. That said, Eleanor Bailie has done a reasonably successful job of ranking the works of Chopin in her book that is often cited on this forum.
I think it makes more sense to look at a work one wants to study/perform and decide, based on one's own skills and potential, whether that work is within one's grasp and will be worth the time and effort required to bring it to performance level. In the long run, does it really matter whether someone has rated that work as difficulty X, Y, or Z?