Divide and conquer. You’re talking about three different situations, storage, transport and shooting.
For storage I divide everything by system: Pentax, Leica and MFT, and generic accessories. Each gets its own case; I use a Pelican hard case and two Vanguard soft cases with dividers, and a variety of Domke bags as well as inserts from Topo Designs, Crumpler and Artist and Artisan. Any kind of bag with suitable dividers or inserts would be ok.
For travel I generally decant what I need either directly into a Domke bag, or into an insert and some other non-camera bag. I’d use the Pelican, with padlocks, for any travel where I needed some security. Some of the inserts fit nicely into a handlebar bag for my bike; they can also go into a backpack or suitcase, or a larger travel camera bag.
For shooting I normally work from shoulder bags; I don’t like using a backpack, but the principle of divide and conquer holds either way. This generally involves further decanting a subset of the kit to match whatever I want to photograph into an insert and/or a shoulder bag. I used a Domke F-2 for many years*, and I still have it, but it holds more than I want to carry. If I find myself thinking of using it, it means I’m taking too much, so I think again and take less stuff in a smaller bag.
Fanny packs/bum bags - just say no. A more versatile option is to use a belt, worn like a tool belt or a gunslinger’s gun belt, with pouches that you can swap and change according to need. I have a Domke webbing belt and a couple of pouches, although in all honesty I’ve never used them. Don’t know if you can still buy them but one of the other manufacturers will make something similar. Lowepro had something similar in the Street & Field range.
*When I was travelling quite a lot in the 90s I packed my clothes and most other stuff, including film, in the Pelican and it went into the hold of aircraft or buses. I packed one change of clothes and all my camera kit into the F-2 and it came with me as carry-on. When I was at the hotel or on a train, especially a sleeper, most of the camera kit would go into the Pelican, which I could lock and attach to something, and I’d keep a small working kit with me.
It’s ok to travel light. This is from ‘American Geography’ by Matt Black of Magnum:
“I’m catching the 11pm train from Fresno to Calexico, 438 miles, 10 hours. One backpack with one pair of pants, one long-sleeved shirt, one T-shirt, jacket, hat, four pairs of socks. Panasonic camera, XPan camera, six lenses, thirty rolls of film. From Calexico, I’ll take the bus cross-country, to Bangor, Maine, and back. It’s 3,317 miles, one way. About six weeks.”