This Everest bag has been my 'briefcase' for the last ten years and is still going strong. It is relatively small- no Timbuk2 'meal for 8' monstrosity and screaming velcro patches. Simple- one large cnetral area, some flat pockets on the front, two end pockets. The main area is wide mouth- not a slit with a zipper, just a box with no top. The flap covers the top well, is heavy enough to not flap in the wind, etc. There is light padding for the central area to give it both structure and a little protection. With one camera- Leica, rollieflex, or Olympus M43 setup for me- there is room for a phone, a tire patch kit and basic tools, and maybe a little more. The flap is large enough to allow for having a jacket captured in the top of the bag if the weather changes quickly or such.
And this Jill-e insert fits inside the Everest bag perfectly. I leave the top flap loose but over the opening; you can also zip or flip the top back and tuck it in so it is never in the way.
All in all, this ends up like a $150 thinktank designer-y Leica field bag for $40. With no pretenses and an off-brand label that can be easily sliced off.
I've tried a variety of ways of doing what the OP describes over the years -minus the twitchy fixie
- and this setup just keeps working. Smaller bags make access and getting things in and out tricky in the field. Larger bags are, well, large and annoying. My goldilocks setup for street walking and riding.