no reason to start PS-only plan on 12/31?

Am I reading this right?

SECURE says that if I implement a PS-only plan today for a 2025 plan year, then there's no 5500 for 2025 because it's retroactive and I just check that box on the 2026 5500.  But if I was paranoid and had the document executed on December 31, 2025, then a 2025 Form 5500 would be required?

Correct.  The reason for the exception is you can set up a Plan under SECURE after July 31st, thus making it impossible to file an extension.