I think an important theme of this thread is that experiences differ greatly from one student to another and one school system to another.
Some high school courses provide excellent preparation for SAT Subject Tests. Others don’t. Sometimes it’s worthwhile to postpone a Subject Test until you have taken the AP class in that subject; other times, it’s not necessary. Honors chemistry in one school system can differ greatly from honors chemistry in another school system. Etc.
One question: Some people here have talked about students taking Subject Tests after completing a high school level course in 8th grade. Will colleges even accept scores from tests taken that early?