I could certainly support the idea of a reduced charge at weekends.
One of the big problems I have with vehicle recognition schemes generally (the Dartford Crossing is the same) is that there doesn’t seem to be a way of telling whether you’ve avoided paying if you entered/exited the zone at about the charge boundary times. For this reason I’m not sure how successful splitting the fee on a weekday might be. I do get the data protection issues, but once logged in to your own account I think they should allow you to see times that your vehicle plates were captured.