One of my best high school math teachers was not a math teacher. He was a retired Navy guy who had been a long-time instructor in a technical field.
He broke concepts down very well and for that reason geometry and trig were highlights of my math career.
However, he had a hard time adjusting to the baby-sitting expectations, which was sad. He quit after one year.
The union got to him...It wasn’t the kids...I’ll bet a buffalo nickle on that theory...
The best math teachers are the people who use math in their jobs.
But somebody with a phd in physics lacks the government-mandated credentials to teach math. He doesn’t have a masters in “education” or a license etc.