yep, its often just not much data that the benefit of the higher bandwith can be noticed. If you copy a 200GB video file, the situation is different - as long the ssd controller can hold the speed ( temps are a huge topic for nvme’s ).
I also re-read your post about DDR vs NVME: it is >50GB/s for your 6400’er in Single-Channel , but usually we use Dual Channel Setup ( hopefully ) . So ist more like 100 vs the max of pci4 is ~7.9GB/s ( 4 lanes) . Thus RAM is still much faster and have not the most of the issue of the ssds chips, etc.