I believe I finally completed this piece, which, based on previous feedback, I think is in the general style of a European folk tune hybridized with a march.
Earlier I posted an incomplete version, which you can check here for comparison:
Thanks to the people who responded to the incomplete work, for their motivating reactions. Thanks to Rabbival507 for suggesting a "fast section" - a device I have noticed in folk music. I went with it, which kickstarted me again.
It's 2 violins, viola, cello, 2 unison double basses (pizzicato only), concert flute, bass flute, nylon string guitar, tambourine, cymbal, snare drum, and bass drum.
EDIT: AND TRIANGLE!
A dutifully returning main theme is interspersed with a chorus-like secondary theme, and variations and spinoffs on the main theme.