Hi Billy,
Welcome to the OCF forum and I am sorry for what you have been through. As for stiff neck and tight mouth, tongue, etc, I was given exercises to perform daily that help my flexibility. I used to be much better at doing them the first year or two but not as good lately. I had tongue cancer, with surgery and radiation (no chemo) in early 2020. I experience strong cramps in my neck now (sometimes daily, can skip a few days, started last year) but can still open my mouth enough to get root canals (many since treatment) and move my neck enough to do sports (surf, ski, bike, jog) so I think I am lucky. Getting active soon after my surgery and staying active likely helped too.
Did you get any advice from a speech therapist or oncologist on how to stretch and keep maximum motion? I have read of some people doing oxygen treatments (hyperbaric chamber) with some success.
Best regards,
Nels