I don't have much advice to offer, as from my limited knowledge you have done everything that i would have first said - ie: bile salt binders (colestipol, cholestyramine), anti-diarrheals, and bulk formers (fibre, metamucil).
Have you tried anything like buscopan or bentylol? These are antispasmodics that may help.
Apparently bile can and often does become an issue after a gallbladder removal, and unfortunately can be a life long issue, but that said, it can be managed!
In regards to dehydration, I think you may want to just up your water intake, and try some things like gatorade and even pedialyte... even though they pass quickly, some of it still gets absorbed, just up the intake from the "normal" requirements.
And I just wanted to say, it sounds like your Gastro has given up, but maybe you should seek another. Sometimes a fresh set of eyes can do wonders. Second, Third and even Tenth
opinions are sometimes what is needed! Keep advocating for yourself... as they say the squeaky wheel gets the grease lol
I hope maybe by bumping this up for you, someone who is more knowledgable than myself will reply.
I hope you feel better soon.