What advantage do you expect to get from snappy? Have you tried running your new classifier using the GPT command-line tool? If you want to loop over multiple input files, snappy memory management may create problems, but you can use the working gpt command-line in python with subprocess.run.
Have you tried to run it from command line with a graph?
Would e good if you would try. Attached is the graph file and you should be able to run it with the following command: gpt G:/Data/Classifier/rfc.xml -f GeoTIFF -t G:/Data/Classified/Classified.tif G:/Data/265_2018_scatsat_1.tif
You might need to adapt the paths.