aaah antibiotics, this is exactly what happened to me and my first anal gland experience lol.
I would cut down on the fiber, because as you mention that creates more pooping, which can affect the glands as well.
There is a video I really like from Dr. Becker and I always recommend everyone with this issue to watch it.
Now, I can tell you that when my little guy first had his fishy smell, I had no idea what it was, I took him to the vet and they expressed the glands and the Dr. told me to bring him in every time it happened and she could express the glands for us but the more she did it, the more it happened... so that is when I saw this video
I feed him raw, and that has made a difference as well. Bones are also key, the food I use the most has the bone in it (not powered but ground because I can see the little pieces of bone in it) which helps at the time of pooping to naturally express the glands. I also have some meat that has no bone in it, so I have just recently bought chicken necks to try, tonight will be our first time!
I cannot tell you how full or empty they are supposed to be... but I know that if for some reason my dog's are a bit full, he will sort of chew on them him self and express them... then he will just smell a bit
But I do not express the glands my self any more, nor let the groomers or vet do it. You just gotta watch for any other symptoms to make sure its not an infection.
Here is also a link about anal glands from another holistic Dr. I like to read a lot.