[Geoff Bacon]

A song can exist in multiple collections so why don't you just create a collection for each composer?

Assuming you already have this info in the Composers field then filter on this field to isolate one composer.
Select all the displayed songs (long press one song and then tap the open square at the bottom right).
Having selected the songs, tap 3 dots (top right) and "Create Collection from Songs"

Geoff