It may be easier to try the arcpy.cim.CIMLabelClass() to create a new label class
Essentially I think you can get your layer object and export out a version of the cim object like this.
cimObject = LayerObj.getDefinition(‘v2’)
From here I would use arcpy.cim.CIMLabelClass() to create a new label placement object and set all of the properties to your liking.
Then append this new object to the cimObject label class list.
Something like cimObject.labelClasses.append(new label class obj)
Check the cimObject.labelClasses to see if its in there.
Then go ahead and set its definition like this
LayerObj.setDefinition(cimObject)
This should work, I don’t think you will see it in the Pro interface under the labelling tab but the label class should be added to your layer object.