Culligan two stage softener problems
Your salt could be "bridged" - having solidified in a way that most of it does not contact the water, which would go with "can't see brine" and "using much less salt" as you describe. If you cannot dig down and find the brine, you may need to break that up. I got out of the habit of dumping in all the salt that would fit after I first experienced the joy of that problem.

In my personal experience, it was more cost effective to get a new softener than to repair my old softener. (Note - unrelated to salt bridging.) Your experience might vary, the problems I was having were different (valves would stick) and it was a very old model that nothing "modern" was willing to work with/fit (and the parts that might tediously have been re-used were not very expensive parts in a new setup.)

Moving to a "modern" system with a usage meter rather than timer-only regeneration may reduce your water and salt use somewhat (depends how predictable/consistent your water use is on whatever timescale the timer regenerates.) Those will track water use and then regenerate at night when the "softening capacity" is used up, rather than on a time-only basis without regard to water use. Or is that some sort of early mechanical use-meter?