The root cause for the slow transfer rate is, possibly, the fact that the workstation M2 drives need to do a lot of random reads.

The fast NVMe M2s (that you are most likely using, I think) are advertised with up to multiple GB/s r/w speeds. That is true for sequential reads for big files, but in your situation you will have random reads instead. Random read rates for common consumer/prosumer NVMe M2 SSDs range from 70MB/s to 110MB/s, that is within your rate of 600Mbps. Reviews of SSDs will often include random read speed results which is where I got that range from.

There are SSDs such as Intel Optane SSDs that can deliver random read speeds in the ballpark of around 500MB/s.

Furthermore you state that you connect the drives via USB-C. Depending on what technology is used, USB3.0, 3.1, 3.2 or Thunderbolt, this connection might cause slowdowns as well. Internal NVMe M2 drives (or other faster PCI-e based ones) might resolve the problem.

To prove or invalidate my assumption, you can use the Windows 10 task manager or the performance monitor. The task manager will give you a percentage of how busy the drives are. If the drive(s) in question sit at 100% or at anything above 80%, then they are likely limiting the speeds. On the other hand, if it is idling, then it is not limiting. Disclaimer: I do not know how reliable the busy percentages of the windows task manager are, especially not for external drives.

If it turns out that the drives on the source side are not busy at all, you might want to check the destination side and see how the drives are doing there (you can use the tool iostat for that).

If none of this helps you because you were able to exclude both the drives on the source and the destination sides as being the root cause for the issue, then I suggest you start with basic troubleshooting steps. For example, you can transfer a big file across and see if this transfer suffers the same limitations. You could reverse the transfer direction and copy some of the small files back onto the workstations. If just the reversal leads to much better speeds, then perhaps there is one component that limits only when reading and not when writing, or vice versa.

Or try to rule out some components by attaching devices directly with no extra switch in between or whatever you can remove from the scenario for testing.