> should you invest a given hour into understanding the lowest code layer of framework X, or talk to people in the business domain?
I think talking to people in business domain is the most important thing you can do in SWE or IT in general. The business is the entire reason you write every line of code, the more you understand, the better you will be at your job.
I do find drilling down into lower layers of your software stack helpful, and can make you a better programmer, but in a much more specific way.
> Pick up yet another tech stack or simply get faster at the one you have that is "good enough"?
Both of these are programming skills and less important, IMO. Trends and technologies come and go; if they're useful/sticky enough, you'll end up having to learn them in the course of your job anyway. Tech that's so good/sticky it sticks around (e.g. react) you'll naturally end up working with a lot and will learn it as you go.
It's definitely good to have a solid understanding of the core of things though. So for react, really make sure you understand how useState, useEffect work inside and out. For Java it'll be other things.