Sorry I completely forgot to respond to this.
In my experience Tech Writing has basically two branching tracks for advancement into higher-paying positions. One is the engineering route, where you're expected to have a CS degree and deep knowledge of various programming languages for developer and API documentation. The other is the "creative" route which eventually leads to UI/UX design/writing. Unfortunately my understanding of coding languages pretty much ends at CSS. I've tried learning Python and Javascript, but I just don't have the brain and/or patience for it. I have a background in design principles already, so UX makes the most sense for me.
So of those two branches, I'd say the developer/API route requires much more "technical" knowledge than the UX rote.
I'm using various online resources to expand my UX background, and I'm about to start a Coursera UX course, as well, which will cost me a few hundred bucks but should be worth it. Here's some links:
https://uxwriterscollective.com/
https://www.coursera.org/specializations/ui-ux-design
https://www.coursera.org/learn/ux-design-fundamentals
https://www.coursera.org/learn/ux-design-concept-wireframe
There are other options besides Coursera, of course (zing!), but you may find what you need through youtube alone. Depends what you feel you need, I guess.