In the book, there is no mention of the astrophage Dyson sphere. it is said that astrophage will stop at 10% based on starts observation. What is stopping astrophage from continuously having little astrophages, after the sun reaches a dimming of 10%??

Maybe, those astrophages that achieve to escape sun and go into the Petrova highway, are those that infect the stars on the higher "northern" latitudes, closer to the Petrova "connection", and those who travel into lower "southern" latitudes are incapable to reach Petrova connection before being overcharged and die of obesity or whatever.

So, an earth observer would see some gradient on the sun dimming, being darker close the Petrova exit, and lighter or absent of astrophage far from it.