I am thinking about law school right now, and classics is great prep for that. That is the only humanities that I am considering.

go for it, but do the law on professional basis and the history on volunteer/hobby basis. you wont earn a dime with a history degree, but reading a book is not only helping you develop interlectually, it also given you the knowledge you desire about antics.

I am considering STEM and I want to get better at it. I also want to learn how to read Greek and Latin too.

be sure to know what you can expect when going into STEM. i saw a lot of people crumble under the pressure, cry at the maths and lose sanity over exams and reports. I struggled alot myself, just some weeks ago i had permanent tinnitus from the stress about exams and deadlines. i had to do 2 more semesters than regularly planned (7 semesters regular, i took 9) for my bachelors, then basically did my masters in 5 (4 is regular). so that is 7 years i was/still am living in severe poverty. when i finish my masters i invested 7 years into that even though i planned for 5 years max. doing 2 more years is cost intensive, keep that in mind.

cant say much about pressure and stress in law school, but i guess it is not much easier.