When I pedaled, my legs would hit my bag at the bottom of the pedal stroke, it was not a problem but it was a distraction. Thus, I wanted to push my bag further back to clear my legs better. I was not looking for anything sizeable like a rear rack, just looking for something to hold the bag further back.
My Carradice Pendle on my light touring bike in photo below. Bike was not fitted for touring at the time of the photo, I use four panniers for that, the bag is nice for longer one day rides and brevets for extra layers, food, etc. There is a shim between the stem and seatpost, standard 27.2 seat post. And a short piece of one inch dowel in the stem, sprayed black. Later I put some inner tube rubber over the dowel so that it does not abrade the bag fabric. An old toe clip strap holds the bag to the stem and dowel. And a couple toe clip straps on top of the bag so I can strap a light jacket on top.
Same bag on my rando bike. I have Brooks Conquest saddles on a number of bikes. It lacks saddle bag loops, but it is easy to run the straps around a loop in the saddle springs.
I am sure when you tour with your Carradice it is packed full enough that it does not sag. But since my bag is often less than half full, I put a sheet of Coroplast in it to act as a stiffener so that the bag does not look like a wet pillowcase when it is mostly empty. Initially I used cardboard, that worked well until it got damp, so later used Coroplast instead. At the time of the photo, just one water bottle, some wind pants that I already shed, and a plastic case to protect my banana. The Coroplast is loose in the bag, not physically attached.
I also have a Nelson Longflap, but have not used it for a few years for anything. Nice bag.
Back to the topic of the thread, do you plan to add fenders to your new bike or just keep the bike as minimalist as possible?