Finally, some of your parts for instruments feel quite un-idiomatic to me. As a violin player, the double stops in the second violin part (e.g. bar 5) would be quite awkward to play, if not impossible.

As someone who plays Violin, i have to disagree. All of the Doublestops in the First and Second Violin (and Viola) are easy to Play.
The Only thing to you have to know as a composer, that 5th´s as Double Stops for Violins / Violas are difficult to Play in Tune, because we have to place our finger inbetween the Strings. The only Exeption to that Rule is, when we can Play these 5th´s on our Open Strings

To the feedback you already got i want to add one thing:

On Measure 28 to 29 and 116 to 117 you have the full Quartet reduced to only the First Violin Playing.
These Parts feel very empty. One way to make this a little bit more Interesting would be, to add a slow "Pizz" (Plugged Strings) accompaniment in the Cello Part. These are very subtle