As a philosopher, you define constraints for any good theory explaining what you are interested in, then you go out and search for help in other disciplines.

Subjectivity means to catch yourself in the act.

When you are simply observing your breath, you are perceiving an automatically unfolding process in your body. By contrast, when you are observing your wandering mind, you are also experiencing the spontaneous activity of a process in your body.

Conscious experience as such is an exclusively internal affair: Once all functional properties of your brain are fixed, the character of subjective experience is determined as well.

All attention is introspection.

Consciousness is phenomenologically subjective whenever there is a stable, consciously experienced first-person perspective.

As a first-order approximation, I would say that phenomenality is "availability for introspective attention": Consciousness is a property of all those mental contents to which you can in principle direct your attention.