Image via https://velo-gap.de/2020/05/26/duerkopp-kettenlos-1912-kardanantrieb/ who have lovingly restoreded this specimen into a rideable state
Another option for a bicycle drivetrain beyond chain, penny-farthing, the velocipede or lever-driven bicycles is just put a driveshaft on there, like a car.
There's scant information out there as to how good it is, mostly because by 1920 or so everybody stopped doing it. CeramicSpeed has some kind of vaporware around this for the roadbike I have posted about before but they made the error of trying to do this shit high-tech and with gear changes instead of figuring you could just get good at pedalling your singlespeed so it sucks.
The singlespeed driveshaft bike, all enclosed? Yeah, might be onto something
CVTs for bicycles are available but considering the insane power losses you get they're exclusively E-Bikes pretty much, so no Fixie rider is ever going to touch them.