Everyones favourite topic

I have a 5yo Bichon Frise bitch, and a 4yo Chihuahua cross dog. Both in good general health, both neutered.
Previously, both dogs had been on Burns Original, and doing OK. The Bichon was having issues with tear staining. I clean her eyes regularly, I even changed her bowl, and looked in to her diet after realising Burns had a lot of cereals in. We looked for a grain free food, and opted for Eden.
They have been on it for 7 months now. The chi has always had some anal gland issues, but now both dogs are having problems. Rocky's have been pretty full, and difficult to express and Poppy has never had issues, but has been chewing at her paws, and back end, along with the typical anal gland smell.
NICE! After having them expressed, it was recommended that we change them to Fibre Response; one of Royal Canin's vast array of prescription diets. From a veterinary point of view, I appreciate that they have a wide range, suitable for almost every ailment that can be associated/managed with an appropriate diet. BUT I dont want to jump the gun, because I know its full of added extras that Id rather they didnt have, Id rather explore our options first. Apart from their AGs, they have done great on Eden. Nice weights, great coats, tear staining has improved too.
Im just wondering what people could suggest? Has anyone had a similar issue? Id rather not change if I can avoid it. It seems to be the best food I can get, short of changing to RAW. Im wondering if I can supplement with whole foods, to bulk things up. Ive asked vets, nurses and other experienced owners, but I get different answers from everyone: Add some raw chicken wings a few times a week, add some oats, add some veg... Its hard to know what to do!
I thought it would be a lack of fibre, but Eden has more fibre in that Burns. Unless its indigestible fibre, but how else can I add this without feeding cereals? Could it be the higher protein? ...I hate nutrition, as a student VN, we're taught a certain way, but in reality its swings and roundabouts!