Going into philosophy as a computer scientist

AI is the total no-brainer. Some universities already have projects studying the ethics of AI, from the perspective both of the AI and human society. Questions of AI and consciousness also arise, as for example in Integrated Information theory (IIT).

Another crossover area is human consciousness, aka theory of mind. Despite the fact that many computing paradigms were ripped off from the theory of mind - memory, sensors, neural networks, etc. the philosophical community can be slow in picking up on discoveries and ideas flowing back the other way.

Information theory also springs to mind. What are the commonalities and differences between information manipulated by a dumb data system and the symbolic meanings manipulated by the human mind?

Formal epistemology would be a natural field to go into from the perspective of your technical skills. Something like the ethics of AI and computing more broadly would be a natural field to go into from the perspective of your broader knowledge. You might also look at programs in CS & Philosophy, e.g. at Oxford and Stanford, to see what they do there, or search for people who are professors in both CS and Philosophy.

In any case, most important is to pick something you're passionate enough to work on for your Ph.D. and possibly your career!

By definition any PhD involves an element of philosophical study! Have you considered looking directly at topics in computer science and applying a philosophical lens to them? For example, the epistemology and ethics of human-facing learning algorithms would be hugely timely.

It depends on what your interests are. I think it's important to choose a Phd program to work on something that appeals to you.

Given your background, perhaps something related to philosophy of language, philosophy of mind, epistemology applied to computing, or questions of ethics might be something interesting to work on.