Couple general notes: Use metric modulation notation between 4/4 and 12/8 (i.e. triplet = eighth in 12). Easier to read than doing math mid-sight-read.
Good use of horn pedal in the beginning. Consider a stagger between parts or an offbeat rearticulation to sustain the tone.
Going from 60 to 100 was a bit awkward. Maybe an accel?
The first trip to 12/8 was way more awkward than the other ones, because you have the snare switching times in the middle of a roll. It takes away from the seamlessness of such a transition. On that note, you cannot use half notes in 12/8. Use ties to sustain the beat.
My next personal step would be to elaborate on this one idea a bit more by transferring melodies and registers to feel like a cohesive section. Don't feel limited by the eighth notes as part of a melody; using the same thing in augmentation with suspensions in the normal figure against it can sound really cool especially with harmony changes happening under. You can use prime form (013) to create inversions on the theme you have. Use a refrain from the beginning for another slow section (ABAB kind of) then finish with a more driving similar idea to the first B section.