Hi amnj,
You are correct that it is rare for an all-latex mattress to become saggy. The few previous posts in the past five years I’ve had on the forum of such instances often came down to improper foundations, mattresses that weren’t truly all-latex (polyfoam in the quilt or base), or mattresses that were in fact too soft and not actual depressions. That isn’t to say that bad pieces of latex can’t slip through the quality control of any facility, and while that can happen, the quality control and consistency of latex production is usually much tighter by process design than that of polyfoam. With any mattress sagging (all foams, even latex become softer with use), you’d always want to start your assessment from the ground up, as any mattress has no choice but to conform to the surface upon which it is placed. Whenever I receive a post talking about sagging, I’ll do my best to walk someone through the process to determine the cause of the sag, but without inspecting everything in person the best I can offer is an educated guess.
As far as combinations that might sag more than others, the upper layers of foam in a mattress receive the most use and “wear out the fastest”, and if a product used too much foam of a lower ILD, it certainly wouldn’t be as durable (nor as supportive) as one using higher ILD foams.
As you’re considering a DIY mattress out of separate components that are purchased from one or several different sources, 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).
For those who 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 comfort, firmness, and PPP as a reference point or blueprint and try and “match” every layer and component in your reference mattress as closely as possible or alternatively 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.
I’ll be interested in learning what you end up creating.
Phoenix