The first suggestion I would make is the same one they made which is moving the firmer layer closer to the surface and increasing the support of the mattress. I understand that this may be difficult with your cramped space but it’s the logical first step.
If you do decide to go in the direction of zoning then you are getting into much more complex scenarios that may involve some trial and error. There is more about zoning and some of the theory behind it and suggestions in post #11 here.