Do the weaknesses in Schopenhauer’s metaphysics undermine his insights into aesthetics, ethics and psychology?

I'm not quite sure what you mean by "weaknesses" per se. In my own view, like many other philosophers and the weakness of philosophy as a discipline in itself is that it is the imposition of a subjectivity on the world therefore it can never be as wholly accurate as the empirical sciences..With Schopenhauer this is also the case. His writings are wrought with his own prejudices, biases, grudges at every level (Schopenhauer on women for example, or on existence).

But this is the case with many other great intellectuals; Otto Weininger is also a similarly inclined philosopher. I do not think it undermines his insights on these disciplines however as each of these subject areas are themselves subject to tendency, bias, the personal. They are inextricable linked to our sentience I believe and nothing that human beings do can be wholly objective.. I do not always agree with Schopenhauer on matters such as aesthetics (the idea it is a cessation of the desires, strivings, pains of the ego) as ... more.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.