At this point you might as well get a larger case that can connect a gpu through a normal 16x pcie slot.

This solution doesn't save any space and doesn't look like you can easily use the external gpu with another pc either. Why bother?

While I can appreciate the modular approach and desire to circumvent bandwidth issues when using external GPUs, this is counterproductive - just get a desktop or laptop with a dedicated GPU at that point.

PCIe 4.0 with just 8 lanes isn't nearly as good as it sounds for modern GPUs, it's a tricky (risers are a b*tch) and expensive setup that is bound to disappoint.