Well, that was ambitious. Quite an incredible effort - very well done to you, Sir. Focusing on mix (taking the word "mix" loosely), there are 2 things that slapped me in the face. One is that the piano feels too mono, especially at the beginning and in the end sections - definitely needs something; maybe a little reverb or a widener like Izotope's Relay. The second is the rhythmic part just after 6 mins: it could be made more dramatic in a number of ways - more attack on the bass, staccato cellos, or staccato electric guitar with a bit of gain.