I've had numerous Shelties with gallbladder issues, three of whom have needed the surgery to remove and one waiting to be done as soon as I can get her scheduled, because the amount of sludge in her gallbladder has increased. My best advice to Sheltie owners is no matter what you feed, keep tabs on your Sheltie's gallbladder with ultrasounds regularly. If there's any indication of increasing sludge, scan more frequently...I do every three to six months. For my one girl with a healthy gallbladder, I scan once a year.
I've always been told that feeding a high quality, low fat commercial food is the way to go, although you can do home-prepared with strict nutritional advice. I don't feed raw, so I can't help you there, but I'd trust what your vet says.
A Sheltie prone to GBM (increased sludge) should be taking a good milk thistle supplement such as Denamrin and, if your vet thinks it's needed, prescription Ursidiol. Mine take both. It does help some; others it doesn't, but I'm not going to take the chance if a simple pill might improve them and avoid surgery.
The ASSA (American Shetland Sheepdog Association) has done extensive research on GBM and has good information on its website
here. Good luck with your pup and good for you being proactive to research this!