I've made a few hurdy-gurdies. Plywood wheels are a drag; although they stay round the edge roughness is an issue. Plexi wheels work well enough, but the best I've found was a wood 'tire' on a ply wheel. A thin piece of wood (walnut is good) is bent to fit around the wheel, making something like 1-1/3 turns. Then you scarf off the ends to fit together for an inch or more, and glue the tire on. A large hose clamp makes a good clamp. Once it's on scrape it round.
It's hard to avoid triggering torsional waves in the strings, particularly large ones. These can 'phase lock' with the transverse waves you're trying to generate, and steal all the energy, producing a loud squeaking sound. I suspect that a plain G string would be hard to control in that way. Violins often use a soft damper on the bridge top, particularly with the high E string, which cuts his down pretty nicely. It's not so much of a problem with wound strings as the windings themselves help damp torsional waves.
You might find you'll get more sound out of the guitar if you put in a sound post. It's hard to say; violins use a taller bridge and that helps as well. The bow is driving the string from side to side, and the top of the instrument works best when it's moving up and down. The post immobilizes one bridge foot, and converts a tall bridge into a bellcrank, with sideways motion at the top of the bridge producing vertical motion at the other foot.
On the other hand, that may not be such an issue. Guitars are actually far more efficient at turning string force into sound than violins. If you did bow the strings on a guitar 'vertically' you'd probably have the world's definitive collection of wolf notes. Maybe making it the less effective way is the right thing to do.
The wheel has a tendency to saw through gut strings pretty quickly, so the old hurdy-gurdies used to have some cotton on the string where the wheel hits it. This totally messes up the intonation, which is one reason the 'frets' on a hurdy-gurdy are adjustable. It also eases up any problems with the wheel going out of round, which used to be quite an issue before the invention of plywood. The usual solution to that was to cut the wheel as a section of tree trunk, with the axil running through the center and the annual rings concentric around it. You have to find a wood that has almost no shrinkage difference between radial and tangential or it will check radially. I think they used mulberry.
There are descriptions of a keyboard hurdy-gurdy, called a 'giegenwerke', iirc. No originals survive, but a few years ago somebody made one. I saw a video on line of it, and it's quite a sound. The key to the mechanism is that there is a wheel for every pair of strings, and the wheels are always turning. Pressing a key on the keyboard presses the string onto the wheel so that it can sound; with a smooth round wheel you only need an infinitesimal gap to silence the string. I suspect they used metal strings so that they could do away with the cotton. They may well have had a damper on the string between the fret and the nut to stop it sounding as soon as it was released. The one I heard did not seem to speak quickly, which is to be expected, but for slow and stately tunes it offered an organ-like sonority that was interesting.