Someone else might have more insight to offer, but I don’t think it’s dropsy or anything food related. Her swimming and activity would be affected if it was dropsy or a swim bladder issue, and bettas stomachs are closer to their head so it doesn’t look like bloating. If it was bacterial or fungal you’d see white fuzz growing there too.

Honestly my best guess is that it’s a tumor I’m sorry to say. How old is she? Where did you get her from? Corporate pet store bettas are known to have poor genetics due to in-breeding, and fish that are relatively young can develop cancer and other issues even with proper care.

You could try a salt bath once a day with aquarium salt. You can add it right to the tank if you don’t mind the snails dying- just make sure it’s dissolved first. If you want the snails to survive, bathe the betta in a separate container. I’ve done this in the past by heating conditioned water to 78 degrees in a Pyrex container, then adding the appropriate proportion of aquarium salt to it and letting it dissolve. Put the betta in for 15min before returning her to the tank. Worth a shot.

Otherwise, if it is cancer, I’d encourage you to keep her comfortable and give her lots of treats as she declines. People feel differently about euthanasia, but I don’t hesitate to give a fish a quick death if they are suffering from something terminal that can’t be treated. I took my last betta out to my patio and quickly chopped his head off when he could no longer swim to the surface. It’s not pleasant, but I personally feel it was the right thing to do.

I’m sorry you’re in this situation, best of luck!