That's a neat piece! The developing variation aspect is interesting.
In terms of instrumental writing, I'm not sure of some things, though. At measure 184, the first violin has a tremolo in the lower voice (implying up+down bows or at least some interrupted bowing), and the upper voice is slurred (implying a single bow stroke). The shifts between pizz. and arco (and vice-versa) seem possible, but they won't be a piece of cake. Be especialy careful with the ones from pizz to arco. Too many fast changes between pizz and arco are more typical of virtuoso writing.
I also noticed you have many arpeggios in stacks of 5ths. I understand you're trying to take advantage of the natural tuning of the isntruments, but these aren't that easy when there are no open strings. For example, violins and violas can't play a triple stop with 2 perfect 5ths (you can probably force things and use a "barre chord", but it won't sound very in tune). Cellos can, but it's a bit awkward. They aren't impossible if the notes aren't simultaneous, but it's not that natural (despite what it seems). The "natural" chord voicings in strings are 6ths, not 5ths.
Keep composing!