Fair re your points, easy setup - i buy all my service items from the same dealer in Kidlington as you (Mercedes genuine parts, quite contrary to the service dept, are very competitive). I'm way too tight to pay those service costs though, they seem like a total waste of money to me!
Re Star, they are a specialist (proper one) in Reading which came recommended to me years ago, they have a genuine Star machine and seem to really know their stuff. They have fixed all manner of Mercedes specific ABC/Airmatic issues etc for me in the past. The techs there seem to know the quirks of every model and always seem helpful when i have turned up with a strange issue - once or twice fixing for free as they have known a certain module or whatever just needs a cleanup.
I know that they are a bit far being down in Reading - i used to live down that way hence still would always travel to them. Not sure if there is one up this way too, i am yet to find one. There is a faux "specialist" in Clanfield, and another in Carterton, but when i have looked deeper they seem to actually just be a normal garage who work on anything - that's all i can find up this way so i still travel to Reading with any Mercedes issues.
https://www.starmotorservices.co.uk/