I can see the reasons why you'ld want a degree, both for personal growth and as a backup for when professional flying is no longer an option.
If you're going for personal growth, do something that interests you (and where you have capabilities.) One study not mentioned yet is Philosophy. One dutch pilot (Lucas van Gerwen) went out to study psychology and now is director of the Dutch center (VALK) that treats fearful fliers and a respected scientist in his field. So even studies that don't seem practical at first may help you into a new job.
I would say, go for any (academic) study if you feel the need... "distance education" (Open University) is your best bet, as they are more flexible to fit in your schedule.