Above I asked for a source for the 6% number. I randomly came across a second-hand source in this article that actually used that same number:
https://www.thelancet.com/journals/lancet/article/PIIS0140-6...
https://www.thelancet.com/journals/lancet/article/PIIS0140-6...
"Approval rates ranged from a high of 33.4 percent in vaccines for infectious diseases to 3.4 percent for investigational cancer treatments"
So by that source, I was wildly pessimistic.