Q5 max tire aspect ratio.

Your Q5 comes now with 18"-rims and the correct /60 tyre ratio in order to keep the diameter of the wheel at the right value, what is important for mileage and speed readings), but the ordinary Q5 comes with 17"-rims (235/65/17)
Switching to 235/65/18 will alter the readings and therefore is not allowed (see the conformity certificate)
The only combinations are
235/65/17
235/60/18
235/55/19
245/50/20 (width has no effect on the diameter, but is function of the width of the rim, here 1/2 inch wider)

Comfort beeing proportional to the high of the tyre, the only way to go if you are not satisfied of the ride of the /60 tyres is to retrofit 17"rims with /65 tyres; within a reasonnable mileage, you dealer will probably accept to switch free of costs (my dealer promised this, but I stayed on my 235/55/19, in my opinion the shortest tyre that preserves an acceptable comfort
By way of comparison, my Q5 is softer than all A4 I tested before buying the Q5

little precision : comfort depends on the height of the tyre, not the wheel
I'm surprised that you consider the 18"-rims as not soft enough, because tyres with an aspect-ratio of /60 are nowadays absolutely common on (older) limousines. On his 235/55/19 rims my Q5 behaves largely as smooth as my Saab Convertible (215/55/16). It is an advantage of the SUV's that they are equipped with bigger wheels (and higher tyres) than their limo sisters (an A4 has 225/45 on 18"-wheels, that is two measures shorter and significantly harder)