Do you get this on a full stomach or do you also get it on an empty stomach? It's strange that it's worse sleeping on your left side as many of your organs are on the right side (liver, gall bladder, appendix). The left side other than the stomach and heart. So if laying on your right side is less painful it could be that you're compressing the air in your stomach and the compression produces less bloat and less pain. Or possibly helps move the gas through.
But it really just sounds like you're having a problem with gastric emptying. Since you've been to a gastroenterologist, and they saw nothing, diet is all I have left. Things you mentioned, like dairy, popcorn, peanut butter, etc. These foods are not always easy on folks. Plus you have no gall bladder.
Lastly, the neck pain (especially neck or jaw) can be heart, dude. Definitely worth more diagnostics. Especially if you're doubled over in pain. That's no fun. Before Viberzzi, I got that dow below my belly button. Not fun. Viberzzi is the absolute last thing you'd want to take because it slows the gut and is not for use contraindicated in people with no/malfunctioning gall bladder.
Have you ever tried a laxative? Also a medication called Xifaxin (pronounced ZY-fax-in). It's an antibiotic.
Just some thoughts.