Okay, so I oversimplified my description previously because I was in a hurry and typing on my phone, and thought the idea would come across just fine. The following is how I actually regularly pack my packs whether soft or with an internal frame. It works very well for both shaping the bag and helping transfer load to the hips in soft packs with larger loads.
1) I stuff my sleeping bag into the bottom of the pack, frequently stuffed horizontally inside a stuff sack of its own.
2) Then I divide most of my remaining gear into roughly two groups. Maybe food and clothing on one side and tenting and cooking on the other, whatever works, often mixing soft stuff with odd shaped stuff for packing efficiency. If I want some extra back padding I may fold my tenting/tarping flat and put it against my back with the "tubes" of other gear behind it.
3) I always have some misc crap that doesn't fit or that I forgot to fit into one of the horizontal divisions, or that I want inside the pack, but need ready access too, and it all just gets stuffed on top (e.g. rain gear, extra water bag, whatever).
As to concerns about bigger packs allowing gear to move around and shift. You can overstuff anyting so it is a round sausage. If you use a slightly bigger bag, sausaging is reduced well before stuff starts shifting all over the place. Then, if you don't use compression straps on your bag, use anything you have that's puffy like a sleeping bag or puffy jacket, or fleece and don't overstuff it, use it as filler around everything so everything stays in its loose, well padded space.
Good luck. Have fun. And yeah, if you don't want to have to think about how you pack your pack, don't go quite so ultra-light and get a frame.