At some point in your career, the languages that you know stop being as important. Once you know 5-10, you'll pick up whatever your current company uses. You may have favorites, but beyond a certain point further specialization has marginal benefits.

Cloud architecture:

A good skill to have. Make sure you understand the nuances of distributed computing. There isn't a huge difference between the different cloud providers, though. However, there are enough subtleties with each that some specialization pays off.

Music streaming:

Not a bad business domain for specialization, with very interesting algorithmic knowledge required. There is a lot of hard stuff here. iOS is probably too specific here.

Other stuff:

The biggest demand right now (that I've seen, so YMMV) is for machine learning specialists. They are going for about $2M per in acqui-hires, about 4x standard dev. That is a very hard thing to specialize in without a math/research background, but learning the ins/outs of deep learning architectures can pay off fairly well even without a PhD. Jumping from web dev to that might be tough, but I've also seen some demand for web based ML visualization and interactivity. Might be interesting for you to combine with music.