I've carried all my equipment in backpacks for 15 years or so, usually they're in the ~25L range. I've enjoyed using ThinkTank and Tenba bags. They've spent plenty of time in trunks and overhead bins, and once or twice in the belly of a CRJ. Occasionally I put lenses in suitcases when I exceed the bag capacity, like a 300mm.

When I am walking around, I use the big bag if I need that much gear, or I pack it into a smaller bag, like my laptop backpack or a small day bag, which may have gear wraps or a camera insert.
This way, I separate the transporting and walking around aspects.

I carry my gear using a Blackrapid "side" strap, which I find more comfortable than a neck strap, and have used some cheap neoprene versions from Amazon as backups. I then use "QD" or 'quick disconnect' area plates on bodies and lenses which I can swap between straps easily. Fair warning, the QD parts seem pretty common gun accessories when searching around. Search for "arca swiss QD plate" to see what goes on the camera body.