There will be certainly more with the main girls!
After 0.7 I will start filling out the world more.
The side girls are neat to add a baseline to random encounters, for example the voyeur and sauna events have the side girls + extra scenes with the main girls depending on progress.
This plus additional actions (like train option with Sophie and Jenny) will also be added to every girl eventually.
Due to work I can’t spend as much time as I would love to on the game, but I won’t disregard any early content and revisited and improve eventually