Then I ended up on a bit of this kind of road and that’s when my opinions started to shift.
This may help. I really like my 320d touring, and if you read the reviews or watch the Chris Harris vids you’ll see things like “perfectly matched torque curve” and “fuss free cruise” etc. Now, on a motorway I’d agree, I love my (auto) car for what it offers me there, which is simple, responsive and comfortable cruising without much effort on my part. Now, here’s the relevant bit; as much as those reviews and videos will also talk about “pace”, “acceleration”, “urgency” and “sporty”, whenever I’ve decided I was to safely make progress along a B road for my own amusement, the car isn’t quite right; Although it makes a good attempt it’s not what it’s made for and I ease off the gas thinking that after all I’m a middle aged man trying to have “fun” in a diesel estate. I’m sure a Stig etc could make it do things that I can’t imagine, but it’s not an M3 like yours isn’t a C63.
Your Merc is the same; it’ll be a lovely cruiser, pleasant place to be, looks great and will tow that ‘van with aplomb, but it’s not a B road machine, and never will be. You’ll always find it slight underwhelming in that regard.
TLDR; The Germans have done a great job at building a car to do most things well, but you’ve expected too much from it in that scenario IMHO with or without the House on the back.
And fwiw no, the AMG Line badge didn’t change that any more than the MSport badge does on mine.