Hi Anja,
I would doubt that there is anyone on the forum that has slept on the specific combination of materials that you are considering that would be able to give you meaningful advice and even if they had their own experience would be unique to them and could be very different from your own.
If you are attracted to the idea of designing and building your own DIY mattress out of separate components and a separate cover then the first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
If you decide to take on the challenge then I would either use the specs (if they are available) of a mattress that you have tested and confirmed is a good match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) as a reference point or blueprint and try and “match” every layer and component in your reference mattress (including any springs, any foam layers, and the cover) as closely as possible or use a “bottom up” approach (see post #2 here).
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel” that may be useful as well.
While I don’t think that air is a particularly good support core for a mattress (you can see my thoughts about airbeds in this article) … if you are committed to trying it then rather than using thicker layers of softer foam I would probably lean towards using a more progressive firmness approach which used the Therm-a-rest blown up to be as firm as possible on the bottom (to help prevent sagging in the middle of the mattress as much as possible) and then a 2" - 3" transition layer that was somewhere in a medium firmness range on top of that and then I would sleep on these two layers so you can use your your actual experience with the combination to help you decide on the thickness and firmness of a softer top layer for “comfort” and pressure relief on top of that to “complete” your sleeping system (except for the cover that you would be using to enclose all the materials and components which can also make a difference in the feel and performance of a mattress as well).
Phoenix