What an interesting ensemble. I can't think of another piece for five trombones. I hope the bass part is alright to play but if the bass trombone has F and Eb triggers it should not be a problem. This piece would also sound good arranged for tubas and euphoniums.
I don't know if it was because of the Soviet connection, or the harmonies and counterpoint you use, but I got a flavour of Shostakovich in this music. It reminded me of the slow movement of his Seventh Symphony or the first movement of the Tenth.
There was a lot I really liked, especially how the two first sections were combined at 46-50. Actually both the main ideas in this piece are of good quality; the chorale theme works really well with the third trombone providing rhythmic interest but not spoiling the mood. The more rhythmic idea was also well thought out and you use hocketing very cleverly to get around technical problems. Counterpoint was well used based on both ideas as was the changing bar lengths. The ensemble was also carefully used: although there are few times where less than four players play, you give them all different material so it never feels like a constant wall of sound. Bar 39 is a particularly clever effect with the staggered fps that will work very well.
Really the only thing I can say about improving this is the links between sections. I think the introduction of the second section at 19 was too abrupt; the introduction of the rhythmic motif should flow more naturally out of the first section. I might be wrong and this will be an effective contrast in live performance. But I think you should also make the transition back into the first section at 39-40 smoother as it seems to stop and start again with the bar rest. Also I think it would be very helpful to the players to add more barlines - dotted barlines if you still want to show phrases - where there is hocketing and more complex counterpoint, as the lengths of bars changes frequently. (I assume there is no conductor for this piece).
So yeah, very good work!
PS. you could arrange this for organ too, it would also work well.