I think it's pretty good. I like the style and the modulation (modal).
It's true that so many key signature changes make no sense because the reading is harder. In this case, I would write the score without key signature, marking all the accidentals.
The role of the marimba is nice. Not always all the instruments have to sound. But Perhaps a solution for those thick (and sometimes not playable) chords in the piano is to lighten those chords and complete the harmony with the marimba.
Regarding the voices, they're effective. Take care of the tesitura, it's good, but sometimes the women are in the limits (high A), for a chorus.