You're better off sticking with FAT32.
And your problem with the NSP Splitter script is because you don't know the basics of operating the command line. You need python3 added to your PATH so you can use the python3 command from any directory (folder). The installer will do this for you automatically if you select "Create Environment Variables." Then, for ease of use in your situation, you're going to want to copy the splitNSP.py script to the same folder you have your NSP that you want to install in.
Finally, on the command line you will need to use the "cd" command to switch to that folder. Then you can type your "python3 splitNSP.py filename.nsp" command, but you have to replace "filename.nsp" with the name of your NSP file! And finally, if the name of your NSP has spaces in it (which judging by your screenshot, it does) you have to surround the filename with double quotes (").