This is called short selling, or day trading, and it's used by professionals around the world in the finance industries. Not just for crypto. I made a decent amount of profit when short selling when I first started getting into crypto, as well. I'd use several exchanges, and over time learned which exchanges were a sort of "premonition" for other exchanges... You know, certain exchanges would always show a pump of a certain alt, so I'd know to pick it up on the other exchanges where the swinging value hasn't set yet. The beauty of it is just as you recognized... If you sell a coin for BTC, your BTC value won't change, regardless of how much USD or the value of the coin you traded goes up or down.
If you're lucky, you'll find altcoins with pretty drastic and noticeable differences in value between currencies. It's rare, but there have been times where I've dumped a coin to a large buy order in BTC where the value was much higher than USD. I'd sell my coin to fill the buy order, take the BTC, and then buy USD. I'd then exchange the USD and use that to buy the very coin I sold (for BTC) at a much lower rate than what I sold it for... All within seconds.
I eventually stopped short selling, as it was a full-time thing, and the potential risk for losing out on a botched decision is high. But you seem to have the right idea, and ultimately, you can make quite a decent profit if you have the intuition, which based on your original post, you do.
Good luck, and welcome to cryptocurrency
