This is a very good start, your counterpoint Is very effective for the most part, though a brush up in a few parts definitely is recommended.
Your opening is of course very similar to Eine Klein Nachtmusik, while not a bad thing, it obviously draws comparisons which I think you could avoid by having your initial theme come through earlier. the second theme is nice with some harmonic development, from bar 70 onwards, I really enjoyed that section. be careful with trills in lower and middle registers, they may come out a bit muddy.
My main problem with the movement overall is one of balance, more care could be taken with dynamics, melodic range and feel, especially in the more chaotic opening it feels a bit too busy and confusing, in the development bar 140 feels very out of place and jarring, but that's my personal perspective. and then the rhythmic themes afterwards feel very out of the blue, maybe if you introduced it more throughly earlier it could work more convincingly.
over all you have many impressive things in this piece and it's a commendable first attempt at string quartet writing.
I look forward to hearing more of your writing.