Hi smokeyyank,
Unfortunately it’s not possible to make specific suggestions or recommendations because the first “rule” of mattress shopping (or mattress surgery) is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP (Posture and alignment, Pressure relief, and your own Personal preferences) or how a mattress will “feel” for you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own actual experience (see mattress firmness/comfort levels in post #2 here).
Assuming that the layers you are using to replace the layers in your mattress that have softened or are breaking down meet the durability guidelines here … the type of material that you use would be a preference and budget choice rather than a “better/worse” choice.
The least costly material would be polyfoam which is the most common type of foam used in mattresses. It’s also the material that is used in most sofas so most people are familiar with how it feels in general terms (although there are a very wide range of different firmness levels as well). Memory foam and latex are more premium foams and would be more costly but they can also be very good options although the cost of making a mistake in terms of the firmness of the layer or layers you are replacing can be higher if the supplier you purchase from doesn’t have a good exchange/return policy. There is more about some of the pros and cons of latex vs memory foam in post #2 here but the best way to decide on which type of material you tend to prefer will be based on your own personal experience since you are the only one that can feel what you feel on a mattress.
The mattress surgery post I linked also includes a link to the better online sources I’m aware of for mattress components and materials.
Every layer and component in a mattress (including the cover and any quilting material in the cover) will affect the feel and performance of every other layer and component both above and below it and the mattress “as a whole” so it may take some trial and error to find the type and firmness of replacement foam that works well in combination with the other materials and components that you decide to keep in your mattress in terms of comfort, firmness, and PPP.
Phoenix