We know of ancient humans and modern era apes and for the most part the time between births is regulated by the same mechanisms and the death before adulthood is at about 50%. Speculating that a species evolutionarily closer to us is somewhere between us and species that share a common ancestor with us and them both is a reasonable guess. Your speculation throwing out numbers like up to 60 children per lifespan (completely unheard of among any apes) while claiming that there is no possible way of knowing was a lot less reasonable than mine.