Firstly, I think the audio rendering doesn't do this piece justice. Everything sounds too muddy, and I can barely hear the 2nd violin and viola parts coming through.
As to the String Quartet vs Piano Quintet, I don't see the value of having a piano added since it seems to be doubling what's going on in the strings. Leaving this as a string quartet piece would be more logical.
Your string writing isn't bad...but it's very "basic". Each instrument/part seems to have a pre-defined range, and the music rarely goes in and out of that range. Maybe throw the melody to the cellist in its higher ranges, or have the viola play something other than this accompaniment role:
I also think the transition to the trio section (somewhere around bars 80-82) can be a little more developed, as the trio (bar 83) is VERY different from what came before....just a bit jarring. It might work better, and be a little more interesting, to have that section in a major key. Otherwise, a bit more counterpoint material can help as well. It just sounds very homophonic at the moment.
Last comment: The outer sections could use dynamic and phrasing marks.