I'd persevere with the oat bran (and chicken wing if you're happy to add that). And keep a close eye on her - if she starts licking her back end a lot and looks uncomfortable, then take her to the vet as you don't want an infection starting.
My now 9yo cocker had a similar late-onset anal gland problem, but they were infected as well. Had to have them emptied every week for over a month. and he had them packed once. I've added chick wings to his diet once a week and that seems to have helped. But otherwise, i watch him carefully, and take him to have them emptied as soon as he starts looking uncomfortable. It's working well so far.