I frequently buy cleaning products at the Dollar General, but have found the store knockoff brands don't do much on the buildup in the shower. The name brands have more of whatever. Right now I'm using an Arm and Hammer soap scum with Oxy product - "no scrub" around here means "less" scrubbing. I spray it on the dry wall, getting as much light coverage so it won't automatically run off, then leave for a few minutes. I'll spray again to moisten it, then use one of those white or green non-abrasion pan scrubbers (Scotch makes some, and there are knockoff varieties). The last time I did the big scrub I determined I'd try to avoid it again, so I have an old towel hanging on the rack near the shower and when I turn off the water I immediately give the entire three walls where they are wet a once over. Big swooping motions, it doesn't take more than 30 seconds. The towel dries until the next shower. This seems to be working. I also pull up the mat and drop it in a heap on the tub floor, not letting it sit there and mildew in place. I do still have to scrub the tub, and I use BonAmi to avoid scratching the tub surface.

When we lived in Kentucky where the water runs red after a rain storm I discovered LimeAway. That works on sinks and such. The dry crystal cleaners like SaniFlush worked to keep the loo from turning a permanent dark red. Your mileage may vary, according to the minerals in your water. I was considering a move to New Mexico and learned that most of the homes there have a whole-house water softening system put in to deal with the strong mineral taste in the water and the buildup on sinks and such. There is something about adding salt to the tank periodically, but the water isn't actually salty? I never figured out the chemistry of how those worked.