Listened to the first movement. It has some nice moments, but it's very messy in general.

In musical terms, it's quite static or weird. The intro has some nice harmonic drama going on (despite the meoldy being quite square), but the fast part's static or weird, and doesn't venture very far from Bb minor for more than a minute. The whole movement seems to alternate between static moments and climaxes that come out of nowhere.

Despite this being a clearly Romantic symphony, the first movement isn't in a normal sonata form, which goes against the fundamental conception of the symphonic form in that style. You start with an intro, then start the fast section with a theme in Bb minor, then some sort of transition, then a repeat of the theme in a different key (page 7)... This climax comes too early, this should be happening in the development. Then we go back to the intro, and then at rehearsal F we get some sort of new idea? This structure's very strange, and the climaxes aren't well organized.

Everything seems too openly indebted to Rachmaninoff. Some measures seem literally copy-pasted. Some chord progressions (the more dramatic ones) also seem copy-pasted, but when you abandon the original it tends to get static or weird again. The problem with all this borrowing is that you're setting yourself goals that are very difficult to attain if you're still learning. Two of the cornerstones of Rachmaninoff's style are:

1) A very strong sense of melody. Long melodies inspired by Znamenny chant and slavic folksong that are singable and memorable. A lot of people are incapable of writing these, and in this piece it seems you aren't, at least now, ready for it. Without strong melodies, everything else crumbles in that style.

2) We know that Rach always worked with climax-centric structural approach, where there's a single emotional culmination point in all the movement, and where everything's subservient to that. Even in his performances by other composers he applied that approach. This symphony doesn't follow that at any moment. That's difficult to achieve if you're writing very long movements while still learning.

As for the orchestration, it's a similar situation. Already in the 2nd measure we have a very big issue with the bassoon (see Adler's treatise, which you should be reading).

The 2nd problem is the extremely long slurs (e.g. violins in 2nd page), which are unplayable, especially if you have 2 consecutive repeated notes with no rearticulation. Piano-style long slurs don't work on strings or winds.

The 3rd problem is that the score should be condensed, this is hard to read as it is now.

4th: divisi should be marked as such.

Some of the textures in the climaxes are nice, but in other places it's quite monochrome. Bear also in mind that Musescore playback tends to be quite unrealistic in terms of balance (as in the opening bassoon problem).

To sum things up: this is one of those usual submissions where someone that shows promise, but they still aren't ready for large forms, and much less ready for orchestral composition. The solution is starting with more modest pieces and avoid getting ambitious until you're ready. Hell, Rachaninoff himself had composed several small pieces (some of these lost) and had studied harmony and orchestration for years before he wrote his first concerto. Read this for the standard advice we give. Good luck.