Dear Pal,
If I did understand your idea, you want to play Rachmaninoff until you spew your guts (sorry, could not resist

). Although my initial impulse would be to remove you from that idea, the challenge of creating a Rach path is amazing. I'm in!
That said, I will start with general levels:
1.
Morceaux de salon opus 10 and
Moment musicaux opus 16. Nothing complex, nothing demanding, but already with some rach flavor of language and piano approach (opus 16 number 3 would be my pick out of that).
2. The easiest
Preludes opus 23 and
opus 32, and
Morceaux de Fantasie opus 03. Here the real challenge begins. There is a wide range of problems, so it is very much about every individual pianist. Nevertheless, opus 32 nn. 5 and 12 are favorites, as well as the Prelude opus 3 n. 2 (the one you are playing, right?).
3. The hardest preludes, and the
Etudes-tableaux opus 33 and
opus 39. Real challenges. The kind of stuff you take much time to play well, although the etudes I played never reached something I can call cool. So, as far as Rach is considered, I'm on this level.
4.
Sonata opus 36,
Variations opus 42. The kind of work that many people is afraid of. To me, it is not an interesting challenge, because I dislike the music. But it is probably just me: everybody else seems to love these.
5. Concerti. I have no idea about how hard the concerti really are, but I assume the word "nightmare" is an useful descripition.
Of course, his output have many other things, and I think every level may be break into more detailed stages, but my experience with his music is limited to this.
I hope it may help a bit. Best regards,
Jay.