[_gabe_]

> they want a React developer, or a C#/.net developer who can pump out web APIs in it or something.

All web APIs follow the same basic pattern. The OP said:

>> try identify the 'timeless' things. Software design over languages, algorithms over libraries

If you understand the fundamentals of software engineering, picking up a new framework or library isn't a big deal. It's the same old tropes with some different syntax and domain language. Especially in the world of web development where you're pumping out APIs.