Currently, I'm considering the following: 1. Quantum Computing 2. Probability and Stochastic Systems 3. Operating Systems
I'm also open to hearing other suggestions!
Generically, of what you listed, the ones that are most likely to serve you the best in your professional career are:
1. Probability and Stochastic Systems. It's amazing how many developers run into pitfalls because they fail to understand basic probability. This will serve you well no matter what you do.
2. Operating Systems. When diagnosing performance issues or trying to squeeze more performance out of software, knowing how your OS works and how it interacts with the hardware is going to be of immense value.
3. Quantum Computing. Might be fun but almost certain to be useless for your career. On the other hand, there's nothing wrong with learning something fun.
One course you might not think to take but ought to consider is a technical writing course. You are expected to document the design of what you're doing and, unless you plan to stay a peon forever, you'll eventually become senior enough that you'll be expected to present technical ideas and proposals to either your team or your leadership. Unfortunately, most developers can't explain or write worth a damn (really makes you wonder how they even manage to code). If you learn how to write clearly and effectively, aspects of your career will go much more smoothly.