I'd just keep the 18s if you have any worry over ride. As you have noted it rides beautifully even on run flats. Ride comfort should be one of the most important aspects of a 5 series for me. I prefer the 662m design over the 19" 664 and 20" 759 OEM options too.I've had my G31 for a few weeks now and it's great!
I'm looking at changing the wheels as it's got the 18" 662m at the minute. My original plan was to get the 19" 664M as I'm not sure if the 20" are going to be too bad. It's like driving on a cloud at the moment with the 18's
My problem is I've found a set of mint 20" with almost new tyres and tpms for not much more than new tyres. I'm just worried I'll hate the ride difference after having it with the 18's on. I suppose my question really is 19's or 20?
Seen a few on owners groups complain around tyre bulges on the 20s, the profile is a bit low. 19s are probably fine, F11 gen also had 19s.