Other than the volume knob, has the touchscreen's responsiveness and menu system been improved?

 

This will be difficult to determine as the 2018 just came out, and those with a 2017 probably won't test drive the 2018. You will need to determine this for yourself.

I tried the 2015 touch screen and could not reliably and safely adjust the volume from the touch screen. The touch screen was not sensitive enough. I hated it enough that I bought the non-touch screen version, which works well. Go for a test drive and try.

Others have said it does not matter. There is a volume control on the steering wheel, and passengers can take their time futzing the volume control at their leisure.

 

It's a feature as passengers shouldn't be futzing with my radio...

I don't mess with the radio enough, that I don't mind covering up the controls a bit by having my phone there: Phone mount

 

This! If I’m driving, I decide the radio volume, not you. I use the steering wheel controls.

That said, touch screens have no business in cars as, by design, they require you to take your eyes off the road to use them.