What you're doing sounds good. There are also other exercises:
https://www.trainingpeaks.com/blog/t...-for-cyclists/
The kettleball workout would be good for your back. My personal experience with ordinary back pain is to work it until it hurts, then rest it, repeat.
Also follow the links at the bottom of the page.
You might also want to read this thread:
https://www.bikeforums.net/training-...e-athlete.html