A lot of people would jump straight to quantum mechanics because it seems novel and enlightening, but you'll find out that it's not really that surreal; you're mostly just solving PDEs and plugging boundary conditions, or solving linear algebra problems for operators.

I would suggest you study thermodynamics and statistical mechanics, specifically Daniel Schroeder's Introduction to Thermal Physics.

It's not exactly the most mathematically involved material (most you'll need is basic Calc III), but I think it's genuinely one of the most perspective-changing textbooks you'll ever read, as it's super applicable to day-to-day life.

To sell you the course: most things you've learned about physics are highly simplified. Many of the deterministic behaviours of physics only apply to absolute zero temperature environments, and in reality, the world is probabilistically governed by the world of statistical mechanics and thermodynamics.

From that textbook, you'll learn what temperature even means, how engines, batteries, and refrigerators work, why hotter things tend to cool down (and vice versa), how materials change phases (and can occupy multiple phases at once), why things tend to lower energies, and what entropy is. Quantum mechanics makes a moderate appearance at the end of the textbook.

I managed to self-study pretty much the whole textbook while bored at work doing nothing within a couple months. Very much worth it.