Couple of good points in there. I’ll answer your actual question first, re versatility: Simply, it depends. Sometimes yes, sometimes, not so much. To illustrate, once you have a system for numbers, you inadvertently have multiple systems. I started (way back when!) with a major system of 1,000 nouns. This is of course well and truly refined now, but we are talking about 20 odd years ago. Anyway.
1,000 nouns based on the major system. Not only did I have an easy way to chunk any of the first 1,000 numbers into 3 digit pieces of information, I also had 1,000 pegs to hang to stuff on. I also realized I could use the first 52 of those nouns to represent a deck of cards. So, rather than learn a memory palace, I would often just use my peg system to memorize whatever I wanted. If I had to memorize a text on chemistry (I was still studying at the time) I would commit each salient point to another peg.
Now, this might seem like I am saying yes, you can have a single system, just because I did it for a time. But it comes with a caveat, which nicely goes back to your first point. The whole point of mnemonics (for me at least), was take something from short term, to long term. But you could simply do that with a decent SRS (which, if you are not using already, you should check out). A technique does facilitate this process nicely. And whether or not you need several ones depends a little bit on you personally, and your goals in life. Let’s imagine you are reasonable athlete. Good overall fitness, good hand-eye coordination. You could, in theory, go around a golf course and get a respectable score using nothing but a putter, you could. But, imagine how much better a score you could get, by learning how to control a driver, when to use a sand wedge, and really mastering that putter on the green! You are far better off putting in the time to learn and master the different clubs to be the best golfer you can be, than simply hacking away with a single club. Even if you can get away with that single club, you could do a lot better with the right club for the right situation.
Similar with techniques. Personally, (and again, this is just me) I’m a life long learner. 20 years later, I have far more in depth techniques, very bespoke to myself, and I actually really love the process of learning how to learn. How my own mind works I find fascinating. So, I have a few different systems and it it’s a matter of the right tool for the right job.
I guess in short, you’ve certainly lost nothing by at least familiarizing yourself with the techniques out there and the way that some people are going about things. If you have a bit of time pre-college to invest in your own ability to learn, it can only benefit you.