Nice job! This seems like a true labor of love. I have four general comments.
1. Structure: Rachmaninoff was exceptional with structure. You see it in just about any of his works so I'd suggest you model that structural strength. For example, how the themes develop, stay just the right about of time, and know how to deliver a knock out blow when needed (at the finale). So in your music, if one only listened to a randomly selected minute excerpt of it, it would be hard to know if that was the beginning, middle, or end. This demonstrates a weakness in structure and development so I would certainly advise focus effort in this area. Usually the way this is taught is by mastering smaller forms first such as 8 bar melody, then song, then binary, then ternary, rondo, variation, sonata, etc. This is certainly not the only way to learn it but a traditional way. Have you studied composition formally? If not, you might enjoy private lessons with a local composer who can work with you on analyzing structure of your favorite pieces. You need structural growth - where are the contrasting sections, the development, the big recapitulation sections? Mastering this will help your music.
2. Piano: others have pointed out and I will just reiterate the piano writing is tedious mostly relegated to up and down scales. The role needs to be more prominent - melodies, perhaps variations on the melodies, etc. and the exception should be the run up to important segments. For example, the run could conclude a cadenza with a gigantic multi octave run that leads a significant harmonic moment. Just as a further example, the run might be in a dominant key where you are really emphasizing the arrival at a tonic at the conclusion of the big scale. This is a very common feature of Romantic/late romantic music because it serves to heighten the harmonic tension but taking longer to resolve so when the resolution does arrive, it is even grander. The point, reserve those big piano scales to important moments because otherwise they will wear out their welcome.
3. Harmony: I would suggest more innovative harmonies. Even Tchaikovsky, and Rachmaninoff would use chord substitutions and dissonances more. You might want to read the Tchaikovsky book on harmony - its well written and basic but does also cover more advanced dramatic devises such as dissonance, modulations, substitutions, 7ths, 9ths, etc.
4. Dramaturgy: This is just a general comment about knowing how to tell a story musically. Part of it is related to form, but it also includes orchestration and just a general sense of drama. For example, your orchestration is a bit heavy - so the grand tutti passages lack impact because the whole thing feels grand. Remember in Rachmaninoff piano concerto no. 2, the big sweeping theme is really just at the end. Similarly, remember how Beethoven Symphony No. 5, reserves the trombones until the final minutes or the chorus and percussion of Symphony No. 9, until the last movement. These are examples of dramatic flare. In general, your first movement should pose a problem that is resolved in the final moments of the last movement. You can allude to the ending by false ends - Rachmaninoff, Mahler, Tchaikovsky, etc., Shostakovich, frequently do this...and only at the end does the false ending actually result in a grand finale. This is a nice way to tie in the movements and also make the ending have a resoundingly conclusive feel that all great dramatists have mastered.
Overall, really nice work - I hope I'm not sounding critical because these are very common, challenging problems that many great composers spent their entire life struggling to conquer. You're in good company.
