I know this is old and whatnot (and not exactly sure if it's a question), but after searching, I had success with this command for --onefile
:
pyinstaller.exe --onefile --windowed --icon=app.ico app.py
Google led me to this page while I was searching for an answer on how to set an icon for my .exe, so maybe it will help someone else.
The information here was found at this site: https://mborgerson.com/creating-an-executable-from-a-python-script