If you have not already...ditch bar soap for liquid.

The last person to shower in the morning takes the used towel and wipes down the inside of the door...max 30 seconds? We have hard water build-up even with a water softener, but the glass door has been installed for almost two years...I have not had to do anything other than the daily wipe-down.

There really isn't anything labeled for "soap scum" cleaning that is truly marble safe. They are all acid based instead of pH neutral as stone requires. The best approach is preventative, as outlined above. The big one is to eliminate bar soap and many hair conditioners from your personal care routine. They will cause buildup on surfaces that can only be safely removed via old fashioned scrub brushes and manual effort.