I would argue that math skills correlates with many facets of general intelligence, and that getting all the way up to "AMC10-acing levels" might not prepare you for calculus, but you'll probably pick it up much faster due to your increased ability to think flexibly and pattern-match, which is really useful early on when first picking up on tricks for differentiation/integration, etc. Learning new material and genuinely learning to think are two different tasks, and I myself value the latter over the former, especially seeing as getting better at the latter makes it easier to do the former. At AMC/AIME levels of math, I would say raw intelligence is a lot more important than real math knowledge, and I value being intelligent extremely highly, so it was easy for me to see the allure of AMC/AIME. Naturally, if that's not your thing and you just value math for the sake of real-world applications (which is an entirely defensible position), then yes, I agree that the AMC/AIME are not that useful. In terms of increasing fluid intelligence, though, I'd recommend it wholeheartedly.
(Note: The final statement in the above paragraph about increasing fluid intelligence is not supported in the academic literature as far as I'm aware, and is purely anecdotal advice to which the usual disclaimers must be applied, etc.)
I've had the same experience with studying math intensely feeling like it buffs fluid intelligence, and then I hear from many sources about how skill training is domain-specific and doesn't transfer well (1), which would mean that's impossible, and I'm stuck hoping that intensely studying math for several hours a day is something that's fringe enough that it wouldn't be picked up in research.
But, if the fluid-intelligence buff comes from studying consistently and at the edge of your skill level, then going through the two AoPS algebra books, but not gettin...
Your job, should you choose to accept it, is to comment on this thread explaining the most awesome thing you've done this month. You may be as blatantly proud of yourself as you feel. You may unabashedly consider yourself the coolest freaking person ever because of that awesome thing you're dying to tell everyone about. This is the place to do just that.
Remember, however, that this isn't any kind of progress thread. Nor is it any kind of proposal thread. This thread is solely for people to talk about the awesome things they have done. Not "will do". Not "are working on". Have already done. This is to cultivate an environment of object level productivity rather than meta-productivity methods.
So, what's the coolest thing you've done this month?