On the way to a fully booked Dansai and also as a cheap option before heading up to Phu Hin Rongkla we found the last available rooms at the Phet Garden:
maps.app.goo.gl
Cheap but basic with no food option.
They have another one but this seems new and of course was fully booked for Phitakon festival:
Phet Resort · Nakhon Thai District, Phitsanulok
They have another one but this seems new and of course was fully booked for Phitakon festival: