My theory on why astrophage dims a star only so much plus how/why astrophage spores
We know astrophage's life cycle. it harvests energy from the local star, follows that star's north pole some arbitrary but presumably relatively consistent distance away from the star in order to find a planet with large amounts of CO2, flys to said planet to collect missing resources, breeds, then returns to the star. My theory is that once there is enough astrophage present, they start to get in the way of each other at the find the planet step. Once that happens, the astrophage closer to the planet will launch toward the planet sending it's energy into the astrophage behind it, which in turn sends that astrophage careening away from the planet and star.
Assuming the energy being sent into the second astrophage from the first astrophage is enough to accelerate it to relativistic speeds, it would never find the planet and instead be launched into interstellar space. Now all it needs is to survive long enough and be lucky enough to be sent in the right direction to eventually find a new star system to start breeding again. As more astrophage are present in a given system, more astrophage is being launched into interstellar space, which eventually causes an equilibrium to occur of how many actually make it to the planet and how many get ejected. And with trillions of astrophage being launched in random directions, one of them is bound to get lucky and be sent to the next star system.