I think you nailed it, in that Heinlein wrote (at least) two very different kinds of fiction - his "juveniles" like Red Planet, Tunnel in the sky, Citizen of the Galaxy, StarShip Troopers, maybe Orphans of the sky, Spaceman Jones, and so, and his heavier work starting with "Stranger in a Strange land", then including things like "Time enough for love", "Friday", "Job: A Comedy of Justice", etc.
Personally, I prefer Henlein's earlier works as well. My advice is to find an adult who is a R.A.H fan and ask advice - or just read the first ten pages at the library before you buy them - or check the original copyright date. Henlein published "Stranger in a strange land" in 1962, and began exploring adult themes after that.
If you want to stick to high fantasy - right makes right, the good guys win -- all while exploring deep issues in philosophy of justice - I'd advise you to stick to book written before 1962.