You probably did it correctly, but you didn't mention "how" you checked the fluid. You have to have the trans warm and running and then check the stick. It's not a bad idea to run the trans through all the gears as well.
If you checked with the engine off, then it would read too high on the stick, even if it does have the correct amount of fluid.
Running low on fluid in the first place might have caused it to overheat and could have fried something inside. The 'excessive' metel on the plug would indicate that something wore out in there........
2013 F150 XLT 4x4 SuperCab EgoBoost Max Tow 3.73 gears OEM 20" wheels.