These have worked for me:

https://www.amazon.com/gp/product/B017R6FJKK/

https://www.amazon.com/gp/product/B07R9VB35J/

Looking at https://www.amazon.com/Profile-PCI-M-Key-Express-Adapter/dp/B07R9VB35J/ref=sr_1_1?keywords=Xiwai+Low+Profile+PCI-E+3.0+x1+Lane+to+M.2+NGFF+M-Key+SSD+Nvme+AHCI+PCI+Express

I don't see a heat sink on that card. Last I checked I was getting at least 1200 on a 1TB Evo Plus in my 4,1>5,1 but I have an interface card which sandwhiches the SSD between two heatsinks.

If the SSD card is running hot it will throttle down to prevent self-harming.

Another idea, check Activity Monitor. Your OS of course use the drive if it is a boot drive and if say Spotlight is indexing files for that drive that would slow your benchmarks.