This is fascinating. I've been thinking there must be some kind of practice/repetition aspect to rules of grammar/good spelling -- like you, I was a voracious reader as a kid and internalized a lot prior to formally studying grammar, and spelling always came easy.
I was always a good speller until the past few years. I've just become more careless in proofreading. Not good.

I believed for a long time that my good spelling tendencies was because of reading alot. This is an excellent reason why we should continue to pay attention to our word spelling and correct grammar: it does model other people's abilities to spell accurately long term.

I also feel that taking Latin in high school for few years, enhanced my spelling abilities since it demonstrates the origins of some English words.

Reading on my own heavily during primary school years, was to help ramp up my English language skills which I learned only after kindergarten even though I was born in Canada.

I was reading adult books from age 9 onward. Of course that was interspersed with books meant for teens.