Is there local pain in the cervical spine in extension or is it just the paraethesia that comes on. are there any positive tension tests eg slump, radial nerve etc.

have u considered scalene anterior syndrome (extension will compress the brachial plexus) this muscle can also cause referred sensations to the arm

why not be wholistic and check the upper thoracics for restrictions too. take a more global approach don't just focus on the c 6 level.

have u addressed other issues like any changes to sleeping postures or pillows

would love to know if this helps