I have 8 way adjustable and heated. Got that also because of my back issues. The problem is no adjustment will prevent you from feeling the thing jutting into your mid-back (other than the after-market back support, which does help).
When did you buy the Avalon? They only recently added the safety feature that causes the problems. I think in the accord people are complaining only about the 08-09 series. Toyotas may not have made the relevant part so prominent, but ever since active head restraints were mandated a lot of people in various makes and models seem to either have neck pain or back pain. When I started this thread, I wasn’t aware of what it was that was causing the problem, but now I know it’s the evil actuator plate associated with the active head restraint. I think given that I can drive it with my back support in it, I’ll wait and see if they come out with a model in the coming year that addresses these issues and would be worth a trade.
And correct, you cannot change seats without losing the side airbag which is built into the seat.