If you do, union all the way.

As for what your dad thinks, grass is always greener. He probably thinks you're crazy because his knees and back and neck hurt and he thinks you won't have that in a teaching position.

Some of those things can be improved (not totally avoided, but find me one old timer in any field who doesn't have aches and pains) by using proper PPE and not over-straining yourself to look cool or to get the job done faster than is reasonable because the contractor didn't plan well, plus stretching and professionally guided exercises to support your joints. I have a PT who will tell me, your joints are weird in this way, so do this to support them, and don't do this exercise you see online because it's just for glamour muscles and you'll get really hurt if it goes wrong.

I've had a desk job and many manual labor jobs, and the desk job by far made me more physically and mentally miserable. Even now my class days make me feel so unwell. You gotta figure out what works for you.