Hi @ruehoudon and welcome to the Head and Neck cancer group here on Connect. After reading all you have been through in the past four plus years, my first thought is "Have you considered going somewhere else for treatment?" On the other hand, you had a very large mass initially, which doesn't bode well for an easy go.
The PET scans will of course be a bit misleading because scar tissue can appear as cancer tissue. Your TG numbers are not usually misleading though. But I'm not a doctor, just a patient. And what you have going on is above my pay grade so I will defer to someone who has been down a more similar road. Likely someone will pop in soon enough. Just wanted you to know this post is being seen and you are not ignored. Courage and hope for a good outcome soonest.