Is introspection a valid type of knowledge Well, I think that like subjectivity, introspection can be considered a filter through which you experience the world, which are by definition, experienced through the lens of your life, emotions, upbringing, what you've read etc. Objectivity is arguably impossible, although the scientific method does attempt to account for our subjective biases and distortions.
I do not think there is a relationship between knowledge being important and engineering as an occupation. I think engineering has financial incentives, but knowledge is acquired through many different occupations, none of them being superior to the other, as can be seen in the raging debates between philosophers and physicists.
I think while rationality attempted to be parsimonious with regards to categorizing "truth" from our own biases, the fact is, this rationality is also traveling through the filters I mentioned earlier. From your point of view, you may have a dispassionate evaluation of "just the facts," but your biases, beliefs, culture, etc, will automatically preclude some "facts" from even being an option to you.
So overall I think introspection is valuable, but I am skeptical that "rationality" and introspection can really be two separate categories when armchair philosophizing. When building a bridge, yeah of course they are different, but when pondering philosophical questions, that becomes much less clear.