Hi Hyla.
Great to see you are making progress, and that you got in touch with Restmore. I think that it is worth visiting and doing some testing as this will clarify many things and set you on a straight course as far as your needs and preferences.
Latex in general is the most durable of all the foam materials (latex foam, memory foam, polyfoam) regardless of whether it is Dunlop or Talalay or is made from natural rubber, synthetic rubber, or a blend of both and I would not have any durability concerns at 40ILD in your weight range and the slab thickness will not affect its longevity. As far as appropriateness the thickness of a mattress is just a side effect of the design and by itself isn’t particularly meaningful because whether a thicker or thinner mattress would be better or worse for any particular person will depend on the specifics of the materials (type, firmness, etc.) and on all the other layers in the mattress.
Thickness is only one of many specs that are used to make different mattresses that perform and feel differently and that makes a mattress suitable for one person and not another. There is more about the effect of thickness in post #14 here. Regardless of how thick or thin a mattress may be, the most important part of the “value” of a mattress is how suitable it is “as a whole” for your particular body type, sleeping positions, and preferences in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) regardless of how thick it may be.
In some cases … lighter weights or people that sleep in “flatter” sleeping positions, have slimmer less curvy body types, or who prefer a firmer mattress such as you will do well with even 6" even though there is less “room” to design in different layers in the mattress. People that have much higher body weights or larger body types may choose more than the “average” thickness and may prefer the feel and extra adaptability of say 12" of latex.
The main benefit of a thicker latex mattress is that it can be more adaptable for heavier weights and multiple sleeping positions. It will compress from softer to firmer more gradually which means that there is more “range” of compression without the mattress becoming too firm for heavier weights (or parts of the body). It can even just be a matter of preference rather than “need”.
I’m looking forward to finding out what you end up deciding after your visit.
Phoenix