I have severe dysmotility. Have you been checked for small bacterial overgrowth? It can be a complication of many disease that affect the nerves/muscles. A patient w a positive SIBO test can have methane producing bacteria, hydrogen producing or both. Methane is often associated w extreme constipation and sometimes gastroparesis. I also have early satiety.

So sorry. I know how "unfun" it can be
Maybe a visit w your GI will lend some more insight?
Good luck!!