Some recommendations that I have found to be personally comfortable on long road trips:
Lexus LS460 (Mercedes S550 is better, but much costlier to maintain)
BMW 3 series/X3 with inline 6
Mercedes E-350
RAM 1500 V8
Current gen Hyundai Sonata (with double pane windows)
Avoid: electric cars (piercing high frequency drivetrain noise), performance-oriented cars (more road noise), SUVs where possible (more low rumble)
Look for: Sedans without hatchbacks. The sealed rear reduces road noise. This is the best form factor for quiet and comfort. Think Mercedes S-Class, Rolls Royce, limousines, etc.
Inline 6 cylinder engines are super smooth and quiet, V8s can also be similarly quiet. A more powerful engine can run at lower RPMs on the highway, creating less noise. A great engine is all but inaudible at highway speed (and even then only a gentle purr ).
Don't bother with sound proofing, it it extremely difficult and it's not just about decibels (quantity of sound) but creating a balanced and pleasing sound signature (quality of sound). That's why electric cars can be so annoying while also being quiet (just like the dentist's office).
Car NVH is science mixed with black magic. A lot of wind noise comes from the shape of the vehicle.