Before moving on to another topic with a different pattern, let me just follow up with some thoughts about goals and progress in the GR/cosmology-focused interest in mathematics.
Unlike wine or music theory, it may seem like there's not as obvious a progression or end-goal, but for me I think there actually is.
For quite a number of years, I've had in the back of my mind a possible (albeit challenging even if just from a lifestyle point of view) goal of doing Part III of the Cambridge Mathematical Tripos. It's a very difficult, taught-Masters course that allows for a focus specifically on GR / cosmology (as well as many other alternatives) at pretty much exactly the level I'd eventually like to get to. (I've long abandoned doing a PhD in theoretical physics.)
Even if I never actually do the Part III, I can use it as the basis for my own private study and progression. It's possible to work backwards through the course prerequisites to a set of Part II courses, then Part IB courses, then Part IA courses.
What's particularly neat about this is I can construct a pyramid of interests so the foundation is fairly broad (although still not all of 1st-year mathematics) and narrows to the specialization I'm interested in as you go up.
Here's what a possible progression might look like based on the current Cambridge Mathematical Tripos:
It's possible to get a syllabus, recommended readings and, in some cases, lecture notes for these.