I recently switched from riding with a backpack on , to a front Wald 198 basket and rear rack .
Happy to get that monkey off my back .
My current everyday carry , is a 12 L x 13 H x 8 W re-usable shopping bag , and I prefer using the basket to the rear rack , though I haven't yet tried panniers , or a transverse saddlebag .
I also prefer 700c to 26 or 650b .
Great idea about getting rid of the backpack. There aren't too many 700c cargo bikes per say, but really any bike could be a cargo bike. From my perspective I wouldn't necessarily call the Wallerang a cargo bike, but it's definitely quite capable.
Most bikes we would consider cargo bikes have smaller wheels and can carry 100lbs and up of cargo. The smaller wheels keeps the center of gravity lower. If you're Load isn't very heavy it might not be so much of a concern.
It sounds like you're needs aren't that extreme and you could fit a rack and basket on most bikes. You could also fit some large panniers like the ones below called Ortlieb Back-Roller Pro. They can handle your bag and some.
From considering some the cargo ideas you'll need to think about the terrain you'll be riding on, distances and budget. Can you let us know some of that?