If you're looking for a highway cruiser, you might want to go older, and look for a W124 or a W126. Unfortunately, they're all getting on in miles and years, but they have a better reputation for bombing down long stretches of highway than the newer models.

FWIW, I've been looking (longingly) at an 05-06 sport sedan to eventually replace my 300E, and driven a couple as loaners. They're fun city cars, smaller than a W210, but not too much smaller than a W124. The non-sport models are more softly sprung, and wouldn't be a bad choice for cruising. I remember thinking as much after doing a 100-mile round trip in a C240. You might see if there is a dealer around you that offers these up for loaners, and take your W210 for some service.

Another thing to consider is whether your issue with the W210 is as much to do with the chassis as it is to do with the seats in the car.