You’d never get Kessler syndrome at Starlink altitudes.

Starlink satellites orbit at around 550km, and get dragged by the little bit of atmosphere that is at that altitude. Each collision might make more debris, but the conservation of momentum means that any debris that gets kicked to a lower orbit will probably burn up on the atmosphere while any debris that gets kicked to a higher altitude will be smaller mass and therefore cause less damage on the next collision after that.

Collisions can still happen, but the runaway conditions where debris begets debris won’t happen at those orbital velocities and altitude.