Great info and for many a rediscovery of long lost knowledge. If I been smart enough, or had a mentor (like you) I would have probably shot my old 12 with 3/4 to 7/8 and not 'needed' to get 20's.
I still have the full choke 12, and should get a reloading setup to make some (they really should make these commercially and affordable). Ive only been messing about with shotguns for the last couple of years as the 12 was just too abusive. After trying, and buying a Topper 20ga. I discovered that those loads were fine, even in that light platform. I since picked up a Spanish SxS and an Italian O/U in 20s, but had learned more about what I wanted; namely, a light upland gun. What I didnt know is if I would like a SxS or O/U more, thus one of each. Each of those doubles are under 6# loaded and a delight to carry and shoot. No, I dont shoot trap, so maybe after 50 shots I would change my mind, but for field and woods, they are as nice as the sgl. shot H&R but with an extra barrel. Truth is though, Ive only needed the second barrel for a missed shot behind a running squirrel, so If I did my part properly the sgl. shot Topper would have been fine.
Along your lines of thinking, I was recently considering making 24 or 28ga. equiv. loads for the 20ga.s
BTW, love your Cherrio reloading component
! We've used slightly mashed Rice Krispies as filler too, it doesnt harden up like Cream of Wheat or Corn Meal.