Sorry your pup is having this problem. I'm sure it's upsetting.
A question and an observation. The observation is that, if the anal glands are becoming impacted from chronic loose stool, the issue wouldn't be the anal glands. It would be the loose stool. What does your vet say about the loose stool?
The question relates to symptoms. What problems do you see arising from the anal glands?
We had a female Basenji who had to have emergency surgery for an impacted anal gland. She did fine with the canned pumpkin to the extent that she never had the issue again. Just to be sure we're on the same page, as I'm sure you know, the canned pumpkin sold for pie making isn't what we want. (I mention this because some grocery stores only sell the canned pumpkin for pies).
I believe that repeated anal gland expression is a route to impacted anal glands.
At six months I'd wait before doing anything drastic, especially since the cause of the loose stools is unknown.