Any experience or thoughts would be appreciated.

Old motorcycle tricks are to degrease & dry the plug and wrench with
gasoline & air. Use a brand new black oxide (not cad or zinc plated)
hex key, and heat the block locally with a heat gun to ~200 degrees.
You might also sand blast the hex key to get even better grip. I don't
think an impact driver would help. You definitely want to avoid
dinging the hex - and that would be easiest with a manual wrench. Does
the plug stick out enough that you can also grab the outside diameter
with a visegrip to get still more torque?

You are right in your thoughts in that you have only one chance before
these things get really ugly. Good luck.