Your story has some simmilarities to mine. I also feel significantly worse when driving because my already tight and huge left SCM tenses up even more. Can you check your neck if one side is bigger compared to the other. My left SCM was injured at birth.
Also felt something pushing on the left side of my throat. Low dose of botox in my left SCM already gave relief.
Your tight neck muscle doesn’t have to be the SCM so have a thorough check.