Hi Michael90,
One of the challenges of choosing a firm mattress and then adding a topper when you can’t test the combination in person is that choosing a suitable topper that is a good match for both your mattress and for you can be almost as challenging as choosing a mattress that doesn’t need a topper in the first place (see post #2 here).
Unfortunately 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 for anyone to know with any certainty which topper would work best for you on your specific mattress so it may take some detective work and trial and error to find the best combination but I can certainly link you to some information that can help to find the “best possible” topper for your mattress.
First of all … post #2 here and the topper guidelines it links to has more information that can help you use your sleeping experience on a mattress as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the best chance of success.
There is 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 (or a mattress/topper combination) 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” and in identifying possible reasons for any “symptoms” you are experiencing on your sleeping system as well.
There is also some information about zoning in post #11 here that may be helpful for those that have more “challenging” circumstances or criteria or if you find that your hips/pelvis is sinking in too far but your shoulders aren’t sinking in far enough (and if any shoulder issues aren’t a pillow issue).
Post #2 here also has some suggestions for a mattress or sleeping system that is too firm and post #4 here has some suggestions for a mattress that is too soft which may also be helpful. I would make sure that any mattress protector or mattress pad or any other bedding that you are using on top of your mattress isn’t interfering with the ability of the latex to contour to your body shape (a mattress pad can add to the surface softness but it can also interfere with the ability of the latex to contour to your body shape and have an overall firming effect) and I would also make sure that the foundation you are using under your mattress doesn’t have any flex or sagging that may be contributing to any issues you are having.
Because you can’t change the mattress you are using … the changes you are making will be limited to changes in the upper layers which will affect pressure relief and secondary support because the deeper support layers and components of your sleeping system can’t be changed so you won’t be able to correct any “symptoms” that are caused by the mattress underneath your topper so “best possible” may be the most realistic target when the type of changes you can make are limited to the top layer.
The choice between memory foam and latex would be a preference choice not a “better/worse” choice and there is more about the differences between them in post #2 here.
If I was in your shoes I would probably be leaning towards a softer topper to “allow” your shoulders to sink in a little more relative to your hips/pelvis but of course this may also increase the risk of alignment issues (back pain) when you sleep on your stomach as well and I would tend to try some of the other suggestions in the posts I linked (including zoning) before exchanging your topper if for no other reason that to see how they change your symptoms and experiences and what you can “learn” from them about a configuration that may work better for you.
Phoenix