Hi Thompson08,
One of the “issues” with an air mattress is that it is always either as compressed as it can be or not compressed at all and there is little progressive resistance to compression between the extremes of it’s compression range. Because of the lack of gradual compression with air … it is much less adaptable to different sleeping positions than any other type of support core. The support core of a mattress (which is what the air bladder is) is the primary part that controls alignment (which is why firmer is better for your back) and the comfort layers over the support core control pressure relief and how much the mattress “allows” your shoulders to sink into the mattress. Trying to control “comfort” or pressure relief with the lower layers is just not very effective for the reasons you mentioned (just like trying to adjust alignment by changing the upper layers is also less effective)
So if I was in your shoes I would set the air bladder at or close to its firmest position (which will provide better alignment and support for your back) and then adjust the comfort and how far your shoulders sink in by using a topper.
The topper that would work best may involve some trial and error and will also depend on what type of layers you currently have over the air bladder. If you have too much soft foam or materials in the upper layers … it can also compromise alignment because it can allow the heavier parts of the body (hips/pelvis) to “travel” too far before they reach the firmer support layer. The key with a comfort layers is “just enough” in terms of the thickness and softness of the upper comfort layers combined with the topper you choose. This will depend on your body type, sleeping positions, health conditions, and personal preferences.
The intelligel is a buckling column gel which has some unique properties as you can read here.
With your weight I would also be hesitant to use memory foam unless it was a higher density (at least 5 lbs) and I would also make sure that you used layers that were as thin as you could “get away with” because visco elastic materials have a property called “creep” which means that the foam can relax over the course of the night and you can start the night off in good alignment but over time your heavier parts can gradually sink in deeper and you could wake up out of alignment.
I would probably also avoid the Serta foam gel toppers because they are a “particulate” type of gel memory foam which IMO would be less durable than other types of gel memory foam or higher density regular memory foam. You can read about some of the different types of gel materials in post #2 here. If you do choose to use a gel memory foam I would tend towards the type that didn’t have particles in it.
A “pure gel” topper will be more durable than memory foam but will also be more expensive.
I would also consider latex (which is a fast response material as opposed to the slow response of memory foam or gel memory foam) because it is more supportive (gets firmer faster) and more durable than memory foam. The choice between fast and slow response is a matter of preference and they have a different “feel”. Fast response materials will be more movement friendly while slow response materials can be more movement restricting.
There are sources for all the different types of toppers mentioned in post #4 here and it may be well worthwhile talking to a few of them to see how each of them may fit both what you need and your preferences of how you want them to feel. It would be worthwhile to talk with each of them that offer the choices you are considering because in the absence of our own personal testing (which is always the most accurate way to know what a certain material will feel like for you) their descriptions, comparisons and suggestions may help you get a “feel” for the differences between them.
The first step though is to identify what you already have in your mattress in terms of soft layering on top of the air bladder which can help you avoid choosing a combination of comfort layers and topper which are too soft and thick and could put you too far away from the support layers of your mattress. In general 2" is a reasonable “average” choice although if your current comfort layers are not too thick and with your heavier weight an extra inch may be less risky than with lighter weights.
Do you know what the comfort layers are in your mattress?
Phoenix