You did it wrong brother.
I agree that the wind on any bike will wear you out. I ride 10 hours from my house in VA to my uncle in TN without any issues. I am on a Touring though. It has an ample windscreen and that is a big plus at my standard cruise speed of 85MPH. I wear a full face helmet on those trips as it cut down the wind noise and I can listen to my music inside of it as I chose to.
That sixth gear would be nice for these long trips but quite frankly, I have never noticed any high RPM vibes of any substance. Maybe you have something going on inside your engine, I hope not but that seems odd to me.
I did add highway pegs and use them and the passenger pegs to move around on my longer trips. Sometimes I have one foot forward and the other back to change it up some.
For the seat, I added a $39 BeadRider. It allows some circulation massage as you move around the seat and most importantly, allow air to blow beneath your bum thereby keeping it dry on those hot days. Worth its weight in gold!
So, before you settle into the Wing lifestyle, (no offense to those who love it) maybe give my ideas a shot and save some money. Of course, swapping your R for a T does involve some cash exchange. Good luck
PS. For any type of physical activity, even sitting on a bike, requires a fair amount of conditioning. You have taken the first step and did a 300 miler; next weekend try another one or a bit further. Baby steps you know, then you are taking down Iron Butts like they are trips to the store…