Love it!
Stuff I liked: the crisp, memorable rhythms - the idiomatic string writing - cello lead-in to bars 9 and 73 - bars 17-20, with great counterpoint in the 2nd violin and a lovely cadence at the end leading into the main idea again - the interplay bar 93 on, plus the lead-in to that section, great stuff - bar 121 is awesome - the pedal at bar 156 - the martial rhythm at bar 41 etc... I could go on...
Stuff to consider: bar 2 and others, the silence at the end of the bar distracts a little, maybe semiquavers A-B in the 1st violin to fill it? The silences work much better in the minor mode section... - bar 2 and others, parallel unison writing in the violins, plus parallel fifths and octaves in bars 36, 68, etc. (which I don't normally mind but in this classical idiom it's best to play by the rules) - the end of the first section at bar 23 could end with a loud mf, then the next section starts quietly at p, for maximum contrast - in addition, I would go straight from bar 24 to 37, the intermittent section just being the same but much lower (a little too much repetition there, I would just cut it) - I would also cut bars 123 and 124 since they work much better in the modified form in bars 238-239 - the ending should just be bar 241 with an ff dominant-tonic cadence with double-stopped chords, as per works of the time; the brief Vivace section is a cool idea but it doesn't hold the high quality of the rest IMO...
Anyway, those are some thoughts. Well done for a good piece!