From childhood, melody has been dominant in our musical lives and to paraphrase an old line, “no one leaves the theater humming the chord changes.” Melody is what the audience understands and most audiences can only follow a single thread of musical thought at a time. That’s why some of the simplest arrangements work. You can do so much with just two lines. Listen to Bill Holman’s writing, it flows, and if you’ve every played them, you know how seamless the individual parts play. Voice leading is there and his melodic statements are complete and completed. Play each individual lead line of your score to see if it flows and makes a complete statement. Are there awkward leaps and skips to satisfy some hip voicing technique? Does it make musical & physical sense on the instrument? Does it support the focus of the arrangement? Many of my keyboard playing students tend to overwrite and over voice because that’s been their whole domain for so many years. Those huge vertical moments don’t necessarily translate to individual instruments. The single line is how they started and it seems to them to be a regression to go back there.
Don’t allow your left hand to be the trombone section. I know those LH (keyboard) stabs and pads are easily and readily transferred to the bones but you’re wasting their talents. I remember a valuable lesson from my Buddy Rich days. Having committed the above sin, one of the bone players on the band called me aside (I think it was Rick Stepton or Bruce Paulson) and said “look at what we can do with this slide.” He proceeded to play fast and seamless melodic lines similar to a sax section and trumpet section. I got the point. I started working on a more linear approach to the instrument. And while I’m at this juncture, the bari player let me know in no uncertain terms that he was a sax player not a trombone. I was doubling the bass bone with bari and giving up a color I could have better used elsewhere.