"I like Aarn’s approach of 2 smaller front packs that don’t need to be attached and detached from at least one of the shoulder straps whenever you put on and take off the pack"

I've eliminated the conventional shoulder straps on my pack so there is no attaching and detaching of the front bag to the shoulder straps. The front bag works for front straps even when it is empty. Straps at all four corners of the the front bag, when tightened, create a very comfortable chest harness that does everything that shoulder straps do. I put the pack on and off by poling my head between the front bag and back bag like a paper delivery person might do with a newspaper carry bag.

"I find it essential that the backpack has a rigid frame that is taller than shoulder height and that the front pack’s weight is correctly transmitted to the top of the frame, rather than the top of my shoulders."

I agree. I try to have the front bag straps running is a straigt line from the top of the front bag to the top corners of the backpack frame. I like them to just kiss the top of my shoulders. All of the weight of the front bag is transmitted to the top corners of the backpack frame and then downward to the waste belt.