Hi and welcome!
I think stock gearing is pretty good for the highway. any lower and you're gonna miss the grunt the bike needs to get going / be fun, even on the road. with stock gearing the R will hold 110 - 115 km/h very comfortably. 120 is a bit more stressed but it can still hold it all day long. (for me the jump between 115 and 120 is quite noticeable in terms of revs, noise and vibration).
it might cruise a bit better with lower gearing, but remember this motor doesn't have much torque down low, so trying to bring the revs down with lower gearing doesn't necessarily mean it'll still have enough torque to pull you along comfortably.
I would recommend you first ride the bike at those speeds before making any decision. 1 tooth up at the front makes a bigger difference than 1 tooth less at the back. for quick changes, it'd be more convenient to switch the front sprocket, which you could change in 15 mins going from off road to road setup.