do you know of any that actually exist? All I can find are active cables that go up to 15 feet which is a lot better than the 3ft of copper but still a far cry short of running to another room easily

I've used Corning's Thunderbolt 3 fibre cable in the 25m length (~80'). It's eye-wateringly expensive, and it works perfectly.

I don't have any personal experience with them, but Amazon and Google are full of options going up to 30m.

I don't see why they shouldn't work; all the translation logic is in the active circuitry at each end, and the low-level protocol and USB-C spec was designed with this use case in mind, so nothing expects a continuous copper connection between both ends. Obviously power delivery doesn't work over that type of cable.