So I speak from experience: stay away from risers! I've had two recently and there were always problems.
Vertical GPUs are cool, sure, but the risers are simply not stable.
It's better to connect the graphics directly to the mainboard.

Does not exist. Because with such extensions the cables become too long to comply with PCIe 4.0.

However, graphics cards that can run PCIe 4.0 also run with PCIe 3.0, as these are downward compatible. You should not notice a loss of performance. https://www.computerbase.de/2020-02...

I have been successfully using a riser cable from https://linkup.one/pcie-4-0-r... for a few days without the GRAKA being downgraded to PCIe 3.0. I received the delivery within a week. Large selection of different variants.