I'm another independent discoverer of something like utilitarianism, I think when I was in elementary school. My earliest written record of it is from when I was 15, when I wrote: "Long ago (when I was 8?), I said that the purpose of life was to enjoy yourself & to help others enjoy themselves - now & in the future."

In high school I did a fair amount of thinking (with relatively little direct outside influence) about Goodhart's law, social dilemmas, and indirect utilitarianism. My journal from then include versions of ideas like the "one thought too many" argument, decision procedures vs. criterion for good, tradeoffs between following an imperfect system and creating exceptions to do better in a particular case, and expected value reasoning about small probabilities of large effects (e.g. voting).

On religion, I thought of the problem of evil (perhaps with outside influence on that one) and the Euthyphro argument against divine command theory.

16-year-old me also came up with various ideas related to rationality / heuristics & biases, like sunk costs ("Once you’re in a place, it doesn’t matter how you got there (except in mind - BIG exception)"), selection effects ("Reason for coincidence, etc. in stories - interesting stories get told, again & again"), and the importance of epistemic rationality ("Greatest human power - to change ones mind").