You might want to also consider a frame bag for some more volume in addition to the bags you cited.
I have a Carradice Nelson Longflap, that is a bit smaller than the one you are looking at. I also have a Pendle saddle bag that is even smaller. I have no experience with the Velo Orange handlebar bags.
I wanted to add some structure to the Carradice bags, initially I put some cardboard in them, that worked pretty well but I wanted something a bit stiffer and more water resistant so I bought some coroplast at Home Despot. I am very happy with that choice. I cut a rectangle and bent it into a U shape so that the corroplast stiffens up the front (nearest to the saddle), bottom and rear. I used a pizza cutter to put creases in the cardboard and also later into the coroplast to aid in bending it where I wanted it bent.
I used the Nelson Longflap on my Pacific Coast tour. The bag sat on my panniers for support, thus I can't really comment on what it is like to carry that large of a bag unsupported since you would not have any panniers. I suspect you would need something to support the bottom of the bag.
My saddles do not have the loops for the straps, but I use a Brooks Conquest on most of my bikes, that is a sprung saddle. I run the leather straps through a loop on the springs. I had almost nothing in the bag on my Pacific Coast trip, but would fill up that bag after grocery store re-supply stops.
I ordered my Carradice bags from the UK years ago, I do not recall which seller I bought them from but I got a much better price than I would have paid from a USA seller. With Brexit messing up the value of the pound, you probably can get good prices by ordering from the UK.
I stopped to make some lunch in this photo, the bag is the Nelson Longflap. You can see how the bag sits nicely on my panniers for support, which worked well for me but not really an option for you.
At one point, we bought a LOT of food, the long flap came in handy, you can see how full the bag became with that extra extension on the flap. The bag is as full as it would get.
The photo below shows my Pendle on my folding bike. Note the white straps on the flap, those are leather toe clip straps that I can use to strap more clothing or other items onto the outside of the bag. I also used a toe clip strap instead of the Carradice strap on the seatpost. You can't really see the straps through teh saddle spring loops that the bag hangs from, but you can see that the bag hangs nicely on that bike. But, that bag is much smaller and lighter than you are considering, so where I did not need any support under the bag, you might. The sheet of corroplast I had in the bag in the photo below is the reason that the bag holds shape well without sagging.
