I am not sure what I use, I suspect somewhere between 18 and 22 gauge wire between hub and headlamp. I want wire that is unlikely to break if it snags on something between the hub and the fork crown. At the fork crown I have some quick connectors that either go to the Sinewave, the headlamp, or both.

On bikes where I use a dyno powered taillight, I usually use whatever wire B&M provides, which is probably similar to your door bell wire. The wire that B&M supplies for taillights and for wiring to the hub is probably equivalent to the wire that SInewave includes with the Revolution USB charger.

If you try to use the frame as a conductor, keep in mind that the headset is between your headlamp and taillamp. Headset may or may not be a good conductor, steerer tube could have some rust on it, etc. I think it is probably simplest and most reliable to just use two conductor wire. Some headlamps and some taillamps are grounded to the frame, some are not. Shimano dynohubs are grounded to the fork, SP is not, I do not know about SON.

I do not recall if you use fenders or not on your bike, is your Secula mounted to a fender? If it is and if the fender is metal, the Secula might be grounded to the frame, I do not recall if the Secula bolt is isolated from the circuit or not. I mention that because if you wire everything up and something does not work, it could be because a couple components are grounded and you got the wires crossed when you wired it. That is how I learned that Shimano hub is grounded to the fork, I also had a headlamp that was grounded to the fork and i had my wires connected wrong so nothing worked because the system was shorting out on the fork, eventually figured that out, switched the wires and everything worked. My first two dynohubs were SP, so when the third one was a Shimano, that was when I learned the hard way that it can get complicated.

I think it would be hard to find a wire that is too high a resistance for taillight, that is a low wattage component.

Most of my wiring projects are somewhat temporary in nature. if I put a rack on the bike and the taillight is on the rack, then the taillight wiring changes, etc., thus the wiring is somewhat temporary and is not carefully hidden. Only one of my bikes has wiring that is intended to be permanent, that one I used heavier duty wire to the taillight, taillight mounted on the rear fender, glued the taillight wire inside the fender where it is hidden, etc.