BMW sunroof issues have been around for a long time. Normally the grease from the factory is adequate. I have an E39 that is 12 years old and the grease still seems fine. I don't use the sunroof but do try to cycle through full slide open and then closed in the summer when the grease is thinner just to keep everything lubed. I never NEVER use the vent position any more due to the age of the arms that lift the rear portion into the vented position being the parts most prone to breakage.
Historically what fails are the arms breaking that lift the rear of the panel into the "vent" position. Pieces get into the track and then next cycle sliding the roof open you get the grinding noise and additional issues. However, I have seen many post on non pano roofs with step by step repair procedures. The parts are available to fix a broken roof and the ones that break are not that expensive. The dealer always opts for the replace cassette for two reasons, one it is easy and two they make more plus they don't have any liability if something else breaks.