Domke and ThinkTank have been my two primary sources of bags, which I have accumulated over many years, and I also have a LowePro or two and I did have a couple of Pelican cases (used for storing lenses) until very recently. For carrying around my Sony RX10 M4 and its little sister the RX100 M6 I have used an old Domke bag that was just the right size. Now that I've just bought a new Sony full-frame camera (A7R IV) and three lenses I'll be figuring out which of my ThinkTank bags or Domkes works the best for them. Each shooting situation is different and locations are different, so when just shooting around home I choose one setup of bag and camera body and lens or two, but when shooting away from home when there might be different requirements, that's when it is good to have a larger bag that holds everything.
When I am setting out to do some specific shooting in the past I have selected the appropriate lenses for that occasion and then have figured out which bag will work out the best for me. I want quick and easy access to the lens(es) and the camera body. Use the gear, then safely stash it away..... hastily pull it out again when an unexpected opportunity presents itself before I've left the place. When going shooting I also use the camera bag as my "purse," so that I'm not bothering with another item hanging from my neck or shoulder. A Domke bag has pockets that will neatly and nicely stash away my wallet and other needed items when I'm going out-and-about to a botanical garden or somewhere else.
I have a ThinkTank backpack but have only used it a couple of times; in the long run I'm just not a backpack sort of person! I like a shoulder bag that I can grab and use cross-body or just on one shoulder and I like being able to reach into the thing quickly, grab the camera with or without lens attached, and then move along..... In the days when I was shooting more extensively and toting around a Nikon 300mm f/2.8 prime, I had a wheeled (Lowepro?) bag to transport camera body and lens and another lens or two without throwing my back out since I was also hauling a tripod with a Wimberley gimbal attached..... Those days are behind me now.
I want to be able to just quickly reach into a bag and grab the camera body if it is not already out ready for action (currently trying out a cross-body sling for the cameras), and I want to be able to easily and quickly change lenses on the fly, sometimes under rather adverse conditions, so a bag that provides me the opportunity to do just that is much appreciated. I dearly love my old Domkes for this reason, as most of them do really well under the situations I am in.