Hi fire_water,
I don’t have enough information to make any specific “comfort” suggestions because there are just too many variables and individual preferences involved that aren’t possible for me to know and “theory” can be very different from someone’s real life experience. I also don’t know the specific “symptoms” you are hoping to correct with your current combination or whether the two memory foam layers worked well for you when they were new of if they were just “better” but still not ideal in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences). I would also consider testing some mattresses with softer latex comfort layers to see if you prefer the “feel” of softer latex over memory foam or whether you want to continue with memory foam comfort layers.
This is probably true. Many of the toppers at Walmart use lower density memory foam which will soften and break down faster than higher density foam. I would use a minimum of 4 lb memory foam and 5 lb memory foam would be more durable yet although it may also “feel” a little firmer (depending on the specific type of memory foam because there can be a fairly big difference between the properties of different types of memory foam). The latex would be a more durable material than the memory foam and it’s usually the top layers of a mattress that will soften and/or break down faster than the deeper layers.
This is also not unusual. The smaller surface area of your pressure points can concentrate your weight more even though you are lighter which can soften or break down the foam underneath them faster than if your weight was more “spread out” over a larger surface area.
If you decide that you prefer to use memory foam then I would use similar layers to what has worked for you in the past (except perhaps choosing higher density memory foam which may “feel” a little firmer) and if you decide to use latex then I would use your experience sleeping on the 6" of latex along with the topper guidelines in post #2 here and the posts it links to to decide on which thickness and softness has the best chance of success (although I would probably lean towards 3" or so based on your experience with 4" of memory foam).
Phoenix