I too have been asking for 10 years for more tracks and trying to find workarounds but at the end of the day I think It's just meant to be a 5 piece band for auto accompaniment.
The new RapidComposer VST is about the best option so far as it will do way more things with midi than can be done in Biab.
When you change chords/scales in it that midi will just fit the new chord/scale.