Charter school migrating off ADP TotalSource — what's the parallel-run reality?

Public charter network (8 campuses, ~550 EE) leaving TotalSource for a standalone HRIS + broker structure at renewal. The PEO has been fine but the per-EE fee is killing us at scale.

For anyone who's done the PEO → standalone transition at a school or charter: how long did you actually parallel-run? What broke first? And how did you handle the 2-month overlap of W-2 reporting between the PEO's EIN and your own?