The market is generally good at setting the market price for rent, it's just that the market price for rent is very high because people need shelter, the location of that shelter matters, and the supply of it at the location you want is limited. As long as rent is set by the market, it will be as high as people are willing to pay, and that price is very high. There's no need for a cartel to overcharge people for rents. The cartel is land ownership. It's built into the system already without any coordination needed.

What is required, as you said before, is decommodification of housing. If housing is no longer an investment vehicle, then its value can be determined by how people are using it.