I have had both the 5 series and the Merc E-Class estates, and the Merc is the comfiest car I have ever had, its not as involving to drive as the 5 series, but its fast, crushes long distances and makes effortless progress, the ride is incredibly smooth, but the biggest difference for me from an MTB perspective is that the Merc has in the real world (not sure on actual dimensions) a taller ride height, and it much better at handling crappy car parks, country lanes and fields - ideally I would have got the E-Class All Terrain but they are as rare as hens teeth, its their version of the all road.
If you are after a proper big estate, IMO there are really only 3 options, Skoda Superb, E-Class, and the Volvo V90 - all of these have massive boots and will fit a bike with both wheels on in the back. I tested all 3 in 2019, Skoda is great and probably the biggest inside but most models FWD and not as good to drive imo, I would want 4wd version plus wife wouldnt entertain getting a Skoda. Really liked the Volvo, IIRC bit smaller internally, tested the Hybrid model and it went likes shit off a shovel, but again wanted to get the Cross Country variant and wasn't able to source one.
Heres my E-Class:
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