Have you tried creating a temporary RAMdisk and using that as the source of your files? A quick search came up with
this list of free RAMdisk software (can't vouch for the quality, of course). That would eliminate any source file bottleneck and let your SSD run free.
As for the Task Manager results, this might indicate that the QLC NAND is the bottleneck, holding even the SSD controller back and leaving it mostly idle. If the SLC cache is skipped for some reason (which is down to how the SSD firmware handles things), the controller would only be doing the work required to write as much as the flash is able to handle, while otherwise waiting and essentially twiddling its thumbs. But as I said, it might also indicate a bottleneck elsewhere (the source SSD is the most likely culprit) or a combination of two or more factors.
Sounds possible, but shouldn't the drive firmware be handling this regardless of platform?