Hi BowtothePhoenix,
In a word … Ouch!
Your mattress tag indicates that your mattress is almost all polyurethane and not latex at all. In other words … the model you have uses a polyurethane core (which is OK but doesn’t have the same value or performance as a latex core) but also only uses a small amount of latex in the comfort layers. The 10% polye… is likely polyfoam or polyester fibers, the 75% polyu… is polyfoam and the 15% blend / inner I don’t know but must be the latex (combined with something else). If this is latex (which is likely) then the 15% is by weight and by thickness would likely be about half of this which would mean in the range of an inch (.75% or 13" = .975").
What this means is that your mattress is likely to be at or close to the end of it’s life and that it’s really not worth doing any surgery or really taking any other more radical measures to extend it because it doesn’t have a latex core anyway and only a bit of latex in the comfort layers.
So a topper may provide some temporary improvement but won’t fix the underlying problem of the foam softening under the areas where you sleep. The areas that are not “used” as much like the edges and middle are still closer to their original firmness level. This also means that the hip symptoms you are feeling are more likely to be from overextension of the joint than coming into contact with a firmer layer (there is probably too much foam in the upper layers for this to happen even when it softens). If this is the case then some of the suggestions in post #2 here may be worth trying before you spend more money fixing the wrong issue, only partly fixing the right issue, or deciding on what type of topper to buy.
Arizona Premium has a 2" talalay topper here (and also carry 3" toppers). If you decide to order one of these … make sure you add a cover as these are “raw” latex.
Sorry to be the bearer of somewhat “bad” news.
Phoenix