Hi Hottubjoe,
I switched your post to a new topic of it’s own because the topic you posted in is a more general topic.
Yes … there are many zoned mattresses of all types (most commonly in latex and innersprings) that have different firmness levels under different parts of the body in either the comfort or support layers. There is more information about zoning in this article and in post #11 here and post #2 here but the best way to know whether any specific zoning configuration is suitable for you would be based on your own personal testing or experience.
The difference between a foundation and a box spring is that box springs have actual springs that flex and foundations have a more rigid non flexing support surface. Most mattresses sold today have a foundation that don’t have springs inside them. Any springs inside your mattress are part of the mattress itself and aren’t a foundation or a box spring.
There is more information in post #4 here that may be helpful if your mattress is either too soft or is sagging but “fixing” a mattress that is sagging or a mattress that is was too soft in the first place is much more difficult than softening a mattress that is too firm because you may need to replace the top layers that are sagging rather than adding something over them or changing the support system underneath them. If your original support system already had little to no flex then it’s unlikely that changing the foundation or using plywood over a foundation that already has very little flex would make much difference and if the plywood didn’t work then other foundations with a solid non flexing surface such as a bunkie board/bed board would be unlikely to work either. There is also more information about the different types of support systems that are generally suitable for different types of mattresses in post #1 here and the two posts it links to in the second paragraph.
If your bedframe is a queen size or larger and lacks a center support rail then there are some suggestions in post #5 here that may be helpful as well.
This would depend on the type of mattress you are considering because there is a difference between a mattress that flexes under the person sleeping on it and a mattress that can bend under an adjustable bed. Most foam mattresses and many innerspring mattresses (pocket coils especially) are flexible enough to be used on an adjustable bed but there are also many innerspring mattresses that can be damaged if they are used on an adjustable bed and bend too much so it would depend on the specifics of the mattress you are looking at.
While it’s certainly true that testing a mattress for 10 minutes can be different from sleeping on it … if you do some careful and objective testing for PPP (Posture and alignment, Pressure relief, and Personal preferences) using the testing guidelines in the tutorial post rather than just testing for a subjective sense of “comfort” or the “showroom feel” of a mattress then the odds are much higher that your testing will usually be an effective way to predict your sleeping experience (see post #4 here.
There is always a break in and adjustment period for any new mattress (see post #3 here) but it sounds like your mattress was already “on the edge” of being too soft for you and that the initial break in and adjustment period probably took you “over the edge” of the firmness range that was suitable for you (see post #2 here).
You can see my comments about airbeds in this article and I agree with you that for most people they wouldn’t be the ideal choice.
You may need both a firmer mattress in the support layers for better support/alignment and a little firmer or thinner comfort layers that are also more inside the range that is suitable for you. There is more about primary and secondary support and their relationship to pressure relief in Post #2 here and post #4 here that may be helpful in understanding what “good support” really means. As I mentioned in the previous links … testing for “subjective comfort” in the managed environment of most mainstream mattress showrooms can have lower odds of success than random chance alone but with more careful testing your odds of finding a mattress that is a much better match for you in terms of PPP is much higher.
I didn’t see any specific questions in your post so hopefully some of the information here has been helpful but if you do have any more specific questions that I can help with I’d be happy to do so to the best of my ability.
Phoenix