You'll need something like EPYC/Xeon CPUs and motherboards which not only have many more PCIe lanes, but also allow bifurcation. Once you have that, you can get bifurcated risers and have many GPUs. And these risers use normal cables not the typical gamer pcie risers which are pretty hard to arrange. You won't get this for just $200 though.
For the chassis, you could try a 4U rosewill like this: https://www.youtube.com/watch?v=ypn0jRHTsrQ, not sure if 6 3090s would fit though. You're probably better off getting a mining chassis, it's easier to setup and cool down, also cheaper, unless you plan on putting them in a server rack.
For the chassis, you could try a 4U rosewill like this: https://www.youtube.com/watch?v=ypn0jRHTsrQ, not sure if 6 3090s would fit though. You're probably better off getting a mining chassis, it's easier to setup and cool down, also cheaper, unless you plan on putting them in a server rack.