DSG fluid is every 38,000 miles, so it's due now. Personally I also do it every 38,000 miles or 4 years (whatever comes first), but Audi do not specify a time limit, only mileage.
Cambelt is every 5 years or 75,000 miles (whatever comes first), so really it should have been done in 2016, but check your receipts. If it's not been done, then get it done. Also change the water pump while you are in there. It's only an extra 10 minutes work on top of the cambelt change and if it fails later you will need to remove and refit the cambelt again, so may as well do it now, preventative maintenance while you are in there anyway.
As for the discs, they don't look too great, but realistically, that is not an MOT failure, so there is no legal obligation to change them. All the MOT states is that they provide sufficient braking force on the brake tester and the left / right side apply equal brake pressure so the car doesn't pull to one side. So I highly doubt you will get anywhere with the dealer regarding the brakes unless they want to offer good will.
The MOT manual states this as the only reason for failing a brake disc or drum based on it's physical condition....
a brake disc or drum in such a condition that it is seriously weakened or insecure
Being pitted with a bit of surface rust won't come under that.