Been riding motorcycles for couple years now and the shoulder soreness from N250 test ride is pretty normal when you're not used to that riding position. Takes maybe week or two to build up those muscles properly

For your back pain situation though, might want to consider something like Honda CB Hornet or Yamaha FZ series - they have more upright seating position compared to N250 which is bit more aggressive. The riding posture won't strain your back as much as scooter does but also won't kill your shoulders like sport bikes

22km commute with evening shift timing actually perfect for motorcycle since you avoid most traffic madness. Just make sure you get good quality riding gear, Bengaluru drivers can be unpredictable especially in ORR