Shooting fish in a barrel doesn't improve your aim

I've been thinking a lot lately about what kind of player I am. It's kind of complicated, but one of my most central questions revolves around how skilled I actually am. It's difficult for me to answer this question in any quantifiable way (as it is for most players). Though my win rate is good in HU and even in tournaments when I really focus, my HU ROI is vastly inflated by game selection, as I really only play losing players, and the stakes that I play at are fish-rich and generally not difficult to beat. My math is pretty poor, and I don't make calculations (aside from figuring odds & outs) in-game. The conclusion that I've come to is that I have a solid understanding of the fundamentals (starting requirements, odds, board texture, etc.), a good grasp of some more intermediate concepts (player types & adjustments, equity, assigning hand ranges, etc.), and fairly good instincts. This level of knowledge is plenty to beat the small stakes, and when combined with game selection you have yourself a money machine. And that's the (sort of) problem.

Having the basic tools to make money beating the fish at the low stakes games has retarded my development and probably made me a worse player in some ways, because thinking critically at 100% focus is not necessary. I can play a very straightforward, ABC game, even relatively passively, and still make money, and as a result my focus and ability to think through every hand in a logical way has atrophied.

For example, I often don't even bother assigning ranges to my opponents when they call my 3bets pre-flop, because so many of them with literally call with 100% of their opening range. While I get a somewhat clearer idea of their possible holdings after the flop, the ranges for loose low-stakes players are often so wide as to be of questionable use in the decision-making process. This has led me to play more instinctively, using board texture, coupled with my opponent's general style, calling frequency, and displays of strength or weakness to judge whether or not I have the best hand at the moment (e.g. he has an underpair or a draw; this is second pair or a float with overs, etc), rather than my hand's equity vs his possible range of hands. I also tend to have a relatively tight raising range and straightforward style based mostly on hand strength, because I play so many loose cannons and calling stations that don't know how to fold. I'm usually thinking on levels 1 and 2, and often on 3, but rarely on level 4 or higher. Again, this is enough to beat the fish, but I question whether this style would serve at stakes much higher than those I play.

I'm not sure if I've explained my concerns entirely or even coherently, but my point is that my default style is definitely exploitable, and I'm not sure I'm ready to adjust correctly to a tough, perceptive, and aggressive opponent. I've seen several training videos for higher stakes HU SNGs, and though I understand the logic and reasoning as they are explained by psimalive and PrimordialAA, I'm not used to employing it in-game, and am unsure if I'm even capable of thinking and calculating through a hand with the speed and accuracy of such professionals.