I think we first need to establish Tom means repeats, as in first / second ending kind, or simply as a repeat using the BIAB "number of choruses"

If it is the first, Guitarhacker's approach is certainly the easiest. It's not hard to add the endings, repeats, DS etc., once the basic linear song is created.

The basic BIAB chorus repeat is even simpler to create, although it has less flexibility. Based on what I see in Tom's initial question is that the song is exactly 20 bars long (not 100). The settings should be 5 / 16 / 3 (to the right of the tempo and key). For more 'repeats' change the 3 to what ever he needs. No need to change or add anything to the chord / melody structure.

What am I missing?

BIAB for Windows Version 2019 (643)
Windows 10 (1909)