Using only one instrument can be pretty limiting, but I feel like you really got the best out of it. Love how it picks up at around 1 minute in with the waltz.
One note on the arrangement, your build-ups are great, but moments where you bring the volume down (like 3:08) don't sound natural. You've got the timing correct on how fast it drops at 2:23, but even then it's audibly clear that the volume is just lowered. I think that lowering each string's volume individually, so they don't all lower at once, might make it sound more natural. You could also try introducing a higher note that can eclipse the sound of the strings as they lower, so the enveloping is less noticeable. Or try some EQ tricks, but I try to stay away from those.
Other than that, the composition is great and lively, well composed, and with a little work could make for a professional sounding piece