I have no experience with PCI-E 5 cables, but I've a PCI-E 4 riser cable from Athena Power that works just fine (and that you can buy right now on Newegg). It doesn't have any special locking mechanism, so I was concerned that it would work its way off of the card or out of the mobo slot... but it has been in place for years now with no problem.
The "away from motherboard expansion card slots feature" isn't particularly uncommon on cases. One case that came up with a quick look around is the Phanteks Enthoo Pro 2. [2] I've seen other case manufacturers include this feature, but couldn't be arsed to spend more than a couple of minutes looking around to find more than one example to link to.
Also, there are a few smaller companies out there that make adapters [3] that will screw into a 140mm fan mounting hole and serve as an "away from motherboard" mounting bracket. You would need to remove any grilles from the mounting hole to make use of this for a graphics card.
[0] https://www.newegg.com/athena-power-8-extension-cable-black/...
[1] https://www.newegg.com/Athena-Power/BrandStore/ID-1849
[2] https://phanteks.com/product/enthoo-pro-2-tg
[3] Really, they're usually just machined metal rectangular donuts... calling them "adapters" makes them sound fancier than they are.
Can't put four 4090s into your PC if every 4090 is 3.5 slots!