I also had fair share of back pain,flying boeing for many years,last was B734.
doing between 60-80 sector a month. After every sector it takes me few minutes to stand straight with pain. Moved to the A320 with more or less the same operation,at the same time I start using an inflatable pillow half inflated positioned to support my lumper and it was like magic that problem and the pain,I would say 80% was gone.
Now flying ultra long sectors with NO problems.
Since then this inflatable pillow is my MEL as a NO go item.
This pillow coast only 4 Dollars,Its worth trying.
I hope this will help some of you guys.
Safe and painless flying.