After mentoring 36 early stage startup (idea-level) to the point of public investor pitches to a crowd, I’d say you hit just about every critical lesson in this article. The only thing I might add is doing live rehearsals in front of a friendly group, multiple times. Ours did one each week for twelve weeks and the improvement was incredible. The program consisted of twelve teams doing a three month program with a new cohort each year for three years.
Regarding the business plan/MBA thing, many VCs want business plans but seldom put much faith in them. They like to see the thinking. The product/market fit you mentioned is probably the most compelling proof you can offer- and the biggest challenge early on.