If you’re having trouble with your lower back and knees, try an Adventure bike. Ok, I’m biased towards the Beemers but there’s plenty of other options. Tiger and KTM as well as the Japanese offerings. If you think they’re slow, a well ridden GS will hammer a moderately ridden crotch rocket in the twisties.Had an 04 stealth black one from almost new (that didn’t have a scratch on, Ian) sold it when my arthritis got too painful & thought my biking days were over, moved house & new route to work had lots of tempting twisties so bought a Yammy 125, problem was it was killing my wrists ringing it’s neck to get the speed for the twisties, bit the bullet & bought another Blackbird, problem is now my knees are knackered & lower back pain.
I can see a time when I will have to let go & my biking days will be over, hopefully that time will be a long way off.
Thing with the GS is that you can spec with lowered suspension if you’re a short-arse. If you’re of normal stature, there’s a thingy under the seat to lower it a tad or raise it a tad to suit your personal comfort.
I know you’re not a fan of BMs but if you want upright with serious performance, then cast an eye over the S 1000 XR. Same engine as the S 1000 RR but obviously calmed down a bit. Wear dark glasses though cos it’s fallen out of the ugly tree and hit every branch on the way down. Hell of a machine though.
Just a side thought too; a significant proportion of people who have lower back pain have it because of tight hip-flexors. They’re attached to the lower three vertebrae and will cause nasty pain in the lower back if they’re too tight. They can be that way for a multitude of reasons, such as posture, sitting all day, lack of exercise, exercising without properly stretching afterwards, whatever. Most doctors don’t think of this so just treat a ‘bad back’.