I had TMJ more than 20 years before cancer, but now it's worse, besides the trismus from treatment, fibrisism when last measured the mouth opening was about 12mm. I've seen several SLP's, been through their boot camp, and did the exercises, which should be lifelong, and can impact eating, speaking, oral care, besides anesthesia options, as you already may know. There is the Therabyte that may help some, but heard Dynasplint is better. I used used my thumb and index finger crossed vs some using stacked tongue depressors, which is better directed by the SLP, if so directed. I assume you were referred to an SLP, and other reasons for TMJ/Trismus were ruled out by your medical team even though out several years from treatment.

Good luck