Arcpy.mp's exportToPAGX is currently unable to export as an overwrite. Use .pagxs to store layouts and need to be able to overwrite with this function.
When attempting to exportToPAGX() with the same filepath as another .pagx in that folder, I get this error:
<module>
layout.exportToPAGX(out_pagx=OutputExport)
File "C:\Program Files\ArcGIS\Pro\Resources\ArcPy\arcpy\_mp.py", line 1526, in exportToPAGX
return convertArcObjectToPythonObject(self._arc_object.exportToPAGX(*gp_fixargs((out_pagx,), True)))
RuntimeError: Failed to export pagx to path: "......."