- Relevant flaw.
I think the answer is different for different writers. I think you need to know some very basics and their desire in the context of the plot, beyond that you need to know what you think you n eed to write them effectively.
I disagree. A lot of writers tend to mistake quantity for quality and start adding really trivial details or focusing on the wrong aspects. As Vilya put it, "over-forming" your character can be harmful to how you write them.
Without knowing the specific writer, and how the character background development is interacting with their work I would be very reluctant to make suggestions on "how much" any writer should know about their MC before commencing the project.
You forgot "relevant strengths". Those are the things that will, in theory, allow the protagonist to overcome whatever obstacles they face. They're as important, if not more important, than flaws.
I disagreed because fantastic asked for advice and saying "it depends" is useless advice.
We're not trying to tell fantastic "the one true way", we're trying to explain the "safest bet we know of on how to write characters"
I think a reason why there are so many not-great writers out there is because so rarely do they get any advice other than "do whatever you feel like doing". There's nothing harmful about setting-up guidelines.
(although for some protagonists, their most relevant strength is more their basic ability to breathe and walk and communicate than anything I'd regard as a real 'strength').
Actually the OP said he knew that there was advice on the subject all over the map, and asked for personal opinion.
I still insist that saying "it varies, do whatever" is bad advice to give a writer and doesn't add anything to writing discussions. I also insist that there is, as you put it, conventional wisdom on what aspects of a character should the writer know.
I also should note that I didn't suggest that "it varies, do whatever", what I said was that after a few key basics, including motivation, there is a huge variation on how much character background a successful writer will develop before writing their work and you need to find what works for you by trying it.