Instrumental and epistemic rationality were always kind of handwavey, IMO. For example, if you want to achieve your goals, it often helps to have money. So if I deposit $10,000 in your bank account, does that make you more instrumentally rational?

You could define instrumental rationality as "mental skills that help people better achieve their goals". Then I could argue that learning graphic design makes you more instrumentally rational, because it's a mental skill and if you learn it, you'll be able to make money from anywhere using your computer, which is often useful for achieving your goals.

You could define epistemic rationality as "mental skills that help you know what's true". Then I could argue that learning about chess makes you more epistemically rational, because you can better know the truth of statements about who's going to win chess games that are in progress.

I like the idea of thinking of rationality in terms of mental skills that are very general in the sense that they can be used by many different people in many different situations, kind of like how Paul Graham defines "philosophy". "Mental skills that are useful to many people in many situations" seems like it should have received more study as a topic by now... I guess maybe people have developed memetic antibodies towards anything that sounds too good to be true in that way? (In this case, the relevant antibodies would have been developed thanks to the self-help industry?)