My configuration is 3" of firm latex, 3" of medium latex, and 2" of soft latex (talalay). I’m certainly open to adding another layer to the top or bottom of the mattress to improve the support and/or comfort.
I understand your point that solid slats should have the same effect, but I only put the solid slats between the existing flexing slats, and only in one region, so I cannot be sure of the load they are actually bearing. Even if the plywood does not fix my back, I’m still worried that these flexing slats (2.75" gaps, 2" wood) could reduce the longevity of the mattress.
I can’t think of any reason why the flexible slats would reduce the durability or useful life of your mattress but they can certainly effect how it feels and performs in terms of PPP.
There is more information in post #4 here that talks more about primary support, secondary support, and pressure relief and the relationship between them and there is also more in post #2 here about the more common symptoms that people may experience on a mattress and some of the possible causes for them that may be helpful in deciding on the types of changes that would have the highest chance of success for you.
If you do decide to add a softer topper to improve either secondary support or pressure relief then post #2 here and the topper guidelines it links to may also be helpful and have a link to the component post that includes the better sources for components and toppers I’m aware of.
Since too much softness thickness in the upper layers of a mattress can be risky in terms of alignment … it may be worth trying an inch of softer latex to start with to see how that works for you and then you would still have the option of adding an additional inch if you feel you need it.