Shimon, it seems that you believe that pashut pshat in mitzvos are childish, and that drush and remez are “mature”, for the big boys….i don’t often say this, but that is a bizayon hatorah. Just because children learn something doesn’t mean it’s childish.
As for being yotzei yeshivas sukkah…this is a pri megadim explaining a bach, quoted in mishnah berurah; he says that when the Torah specifically gives a reason, kavanah is me’akev. That’s by tefilin (lemaan tehiyeh), tzitzis (lemaan tizkor) and by sukkah (lemaan yaydu). Some hold that the bach was saying like you, but the pri megadim holds it’s le’ikuva