Here's my P5800X 400GB anvil score for reference on a 7800X3D + X670 setup. This is with me using it as my main drive so the score is a bit lower than if I were benchmarking it as a separate drive. I got the full 40K when I first tested it before migrating my OS. I hammer this thing by training AI all the time and it's a real champ:
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What slot is the card in though? It should be the top slot. Lower slots might work but it depends heavily on your motherboard.
I personally use the StarTech PCIe to u.2 card and it just works but that might simply because I have it in the top slot with my 4090 in the bottom slot.
In addition, you never specified if this is your OS drive. You are going to loose about 2,000 points when testing it while it's your OS drive and you'll loose additional performance if you have background tasks running. If you haven't already, try to ensure nothing is potentially interfering with the benchmark.
I assume you mean redriver here and not refrigerator. The m.2 adapters with the redrivers may require some fiddling with in order to get the signal to where you want it and the problem is unless you run Linux I'm not aware of any way to look at PCIe communication errors on windows. Wendel from Level1Techs was able to do so with Linux. He did a whole video on getting PCIe 4.0 U.2 drives to work and it's in general a PITA. Hence why I purchased the StarTech U.2 card as it was the easiest and most reliable option among those Wendel tested.