What if to create a Starborn part of the process (or the whole process) is that an Outsider (or part of an Outsider) needs to be joined with a human host/embryo. It makes sense that Starborn can hurt Outsiders more easily if they themselves were part Outsider. Sort of like what makes them strong also makes them weak. If this is true, then it makes sense why people like Morgan were against Harry. Because no one likes Outsiders and being even part Outsider could be seen as a very bad thing to some people. It also makes sense why LTW needs to talk with a lot of people before telling Harry because he needs to understand how/what to tell Harry. Because just telling him (or anyone else), "Hey Harry, you are part Outsider. You know the race of being that no one likes. Well, you are kinda one of them because your Mom and others want to use you for the greater good." Harry would not respond well to that. (Yes, LTW doesn't talk like that, but that's the gist of the conversation). One thing I'm unsure about is how 40,000 kids could be part Outsider. I could understand a few because someone summons the Outsider and then bonds the child embryo to it but having that many seem unlikely. Maybe at certain times, the Outsiders can cross the border of their own accord and choose their own hosts. And there are so few Starborn left because some people hunt Starborn (or Outsider hosts) because of the fear and lack of understanding of what Starborn are or what they have the potential to become.
Starborn are born at the right confluence in time. There's nothing that says a starborn must have talent. If there are 40,000-60,000 starborn at the start of this cycle, that's something like a 4-6 hour window where they could have been born.
If starborn need at least some level of talent- well, we don't have the numbers to calculate that, but it does stretch the window a bit.
Bonus, it should make them easier to find. October 31st birthdays what, 1975? Narrows the pool considerably.
If I were to guess, I'd go with starborn having an ability to redefine reality- and either eject or permanently accept Outsiders thereby. Outsiders are outside reality, which is why magic doesn't do much to them. A starborn forming intent at an Outsider infiltration implicitly makes it part of reality, and therefore vulnerable to reality, insofar as it redefines them from "a projection from outside" to "that which I am punching and thereby is now subject to being punched." I'd also guess it cycled around to Lucifer, the Morning "Star", on some level- the first to rebel and change reality. Maybe starborn are echoes of what he did. It would tie the Denarians in in a sideways manner.