At 28 years of age, who knows what chemicals have been used to clean the bath. And, with all that scrubbing, your probably close to making a hole in them!
At this point, it's only gonna get harder and harder to clean. The 'dirt' is now getting 'trapped' in the fine fibers of the bath. You can use what you're using but clean it more often and you'll do less scrubbing. If this is not an option, replacing the baths may be one way to increase your limited 'free time'. If you're buying cleaners specifically for fiberglass AND soap scum removal, and they're not working, well, I've already said it.