Hello PhilipShutler,
I'm not sure why the CIM value isn't applying, but I do have a potential work around.
If you do the process manually once, you can "Save a layer file" and use "Apply Symbology From Layer (Data Management" to import the symbology (including the sample size setting) to future layers.
Save a layer file
Apply Symbology From Layer (Data Management)
It looks like the "Save a layer file" needs to be done within ArcGIS Pro, but the output .lyrx or .lyr file can be used in a python script and applied to other layers.
Cheers,
Mike