The 980 Pro and the Sabrent Rocket 4 Plus I have in my gaming PC are supposed to be fairly comparable. I can pull content form the NAS at 522MB/s to that machine no problem (from the SATA SSD, so basically maxing out), its sending to the NAS which tends to be the issue.
Copying from the Sabrent to the Intel 660p (high-end to low-end NVMe SSD) in that PC can do 2.1GB/s for a 3GB file but copying to the NAS with the P1 (which is basically the same SSD just different brand) bursts to 2Gbit or so then drops down to barely above Gigabit, though I just double checked and this seems to apply across all drives in the NAS so appears to be a different issue to what I was originally discussing (bottlenecking on copies entirely on the NAS itself).
This is doubly weird as the NAS has 25GiB of RAM assigned to cache right now so it "should" be able to just dump it into RAM at full speed then write it out to permanent storage in the background. Reading back that file does 560MB/s as its probably coming from that RAM cache so happily maxing out the 5Gbit link back to the main switch.
Its not the end of the world, its just rather odd and highlights how networking can be such a minefield of issues. Its never as simple as plug and play, very much the old MEME of plug & pray.