In addition to the booster (it's now heavier, longer, more powerful and better balanced, but essentially the same kind of construction) I'm working on ground infrastructure for loading the previously mentioned shuttle.
It will require modifications to the cargo bay and surrounding, but hopefully nothing too severe.
Task 1, open and close an empty payload bay, is now complete.
Task 2 is be able to dock it in the open position.
Task 3 will be the most difficult one, driving a craft into the payload bay with (ideally) a 100t payload.
I'm likely reinventing the wheel, but I'm having fun doing so.