TL;DR: There's probably not an obvious, "learn this and you'll increase your job prospects" technology for you at this point. But, you can also decide to learn almost anything, and because of your years of experience, open up a ton of new doors.

I'm also a self-taught engineer (well, I've done course work now, but only years after starting in the field).

If I'm reading this right, I think you're in a position where there's not an obvious next thing to study, as you've more or less become an expert in CRUD applications.

I think a lot of developers hit this point, especially us self-taught types. When you're making your way in the industry, it's really easy to notice the in-demand skills/technologies that you don't have, and to add them to your toolkit one at a time. It's a nice, linear progression. Then, you reach a point where that approach doesn't work anymore. If you're a 10+ year developer who knows React and Vue and is strong with vanilla JS, adding Angular to your toolbox won't raise your salary or your skills—you could probably pick it up on the fly if you really needed to.

I think at this point, you need a paradigm shift away from "what is the market pulling me to do" and "what do I want to do." It sounds like you want to stay an IC, so that means it's going probably going to be purely technical—so what technology do you find interesting?

For me it was machine learning, and then hardware-level stuff. That's what lead me to take university classes and study higher math/more fundamental CS. There wasn't an obvious pull in that direction from the market—none of my bosses offered me a raise if I became an ML engineer—but I liked it, and because I had years of experience already, becoming proficient in the field opened up an entire new world of opportunities in terms of career and personal life. I wasn't a new grad who studied ML. I was a career developer who'd built businesses, who now had a background in ML. You could apply this same logic to any new field you study, it doesn't even have to be technical. If you got really into design, you'd be a designer with 10 years of development experience. If you got really into sales, a salesperson with 10 years of dev experience is a hot prospect at any company that sells to engineers (in other words, some of the most valuable companies on Earth).

If you want to stay in web development and level up there, I'd recommend following the advice of many on this post. Pick a part of the stack you find interesting, and dig a thousand layers down. Even better, if you find something you think could be done better than the standard tooling does it, build something there.