Hi rictorjr,
I would keep in mind that if your symptoms are lower back pain or discomfort that your mattress may already be too soft for you (considering your weight as well) since these are most commonly symptoms of alignment issues where the comfort layers are too thick or soft.
I would probably consider trying either the 3" of 19 ILD Talalay or the Dunlop directly on the core to see if your back issues show any improvement (even though you may replace alignment issues with pressure issues).
If you are confident that what you need is even softer yet then the next logical step would probably be a 3" 24 ILD Talalay transition layer. This would also be very similar to the PLB Beautiful mattress (see post #2 here) so you may want to do some careful testing on a Beautiful in a store to see how well it seems to work and feel for you.
Since your other layers are also “natural” Talalay and not “all natural” … I would probably go with natural for the transition layer as well since it will be a lower cost than the “all natural” you are considering.
Phoenix