Michael,
The convention in UK 0-scale is to have bogies with internal functional frame and hang the bogie frames on the outside as cosmetic parts. This applies equally to kits and r-t-r.
In the US and Europe, most of the 2-rail locos have functional bogies, usually sprung and made from cast brass or machined parts. In this case the drive can be axle-hung, perhaps using a shaft drive and occasionally miniature traction motors.
R-t-r in the US has now defaulted to individually driven bogies with rigid functional inners; bogie frames are usually die cast for weight, but wholly cosmetic.
@Yorkshire Dave is currently working in P48 (the US equivalent of S7) and has locos with sprung inner bogies and rigid inner bogies. I've got locos in build with shaft drive and cast bogies, which look and run really well. It would also allow things like rotating bearing caps, should they be needed.
I've also got a couple of UK diesel mechanisms with external functional bogies and shaft drive as far as the design stage.
A builder of your calibre probably wouldn't worry too much about producing a functional bogie in 7mm scale; all the detail can still be lifted from a kit if you want to go this route.
Steph
Red Caboose GP9 kit chassis with PD Hobbies trucks and drive. Top as standard, lower as Dale-ised (ball races, higher quality chain and sprockets, 3d printed gearboxes).