First of all, great arrangement. I'm just hearing some mixing issues that are bringing this down.
The timing seems odd to me. Don't take this the wrong way, but the drums feel a little sloppy in particular. It's as if you played this in live with some Delay Compensation mistakes but then left them in. The brushed snare also kind of sounds more like white noise to me, so maybe a different sample can seem less white-noise-like.
The bass at 0:54 clashes a little with the low notes from the piano. You might wanna high pass the piano above 200Hz. At 1:17, the sax that comes in sounds too upfront and seems fake. It should be quieter with some more wet mix on the reverb to blend it in better, and perhaps a little scooping EQ in the low-mids to let the piano notes come through. 1:54 is just a small thing, but I think a softer cymbal could connect the two sections more cleanly. Same at 2:17. The strong hits there kind of don't connect the dynamics IMO.
2:28 - 2:39 brings in some more realism issues in the lead on the left. Sounds like a violin but didn't play like a violinist would. At 2:51, the low guitar clashes with the bass, and later, at 3:01 the sax's low-mids clash with the bass.
3:50 is a very minor thing, but that low piano note suggests to me that perhaps you could keep experimenting with different piano samples until you find one that works well for all the notes you're playing. It doesn't sound that fake to me, but it sounds thin. The other, higher notes sound good IMO. Odd thump at 4:22, whatever it is. Might be an acoustic bass?
Overall I like the arrangement (good concept, nice dynamics and flow), and the main issues I would address are the low-mids mixing, the sax volume and EQ, the sequenced violin realism, and the little thump at 4:22. Good job so far.