Struggling With Low Speeds < 15MBps Over Gigabit

Jumbo frames clearly aren't working on your connection (even the standard 1500 byte frame size is not working) . There is no point in testing them unless we can fix that. I suggest you give up on them for now, as you have a more basic performance problem with NAS writes that you need to figure out.

Also, the LAN test results appear to be wrong, since your drag-and-drop measurement is 3x faster. Try the NAStester and let us know what it reports in both directions.

Can you check the NAS network stats, and see if you are getting transmission errors when writing to the NAS? This is via Frontview\network\interfaces. Click on "show errors before the test, and save the result. Click again after the test so we can see if anything bad is happening during the test.

I also strongly recommend re-enabling the broadcom NIC, and seeing how the speed compares. Also, try the ping test with the broadcom and see if you can get 1460 working with that.