Hi lallim,
The first place I would start your research is the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and know how and why to avoid the worst ones.
The tutorial also includes some testing guidelines that can help you confirm which mattress is the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) and there is also more about the different ways to choose a suitable mattress (locally and online) that can help you assess and minimize the risks involved of making a choice that isn’t as suitable for you as you hoped for in each of them in post #2 here.
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 (different mattresses will affect the choice of topper that works best) can be almost as challenging as choosing a mattress that doesn’t need a topper in the first place because the only way to know whether the combination will be a good “match” for you will be based on your own personal experience (see post #2 here).
They (or you) could certainly do surgery but once again the only way you will know for sure whether the new design will work for you in terms of PPP will be based on your own personal experience. If you can’t test a mattress in person to confirm that it’s a good match for you then the return or exchange options and the costs involved (if any) would become a much more important part of the “value” of your purchase.
The choice between different types of materials and mattresses would be a preference choice rather than a “better/worse” choice so some people may prefer a latex hybrid with an innerspring and a latex comfort layer and others may prefer an all latex mattress. Once again only your own personal testing and experience can tell you which type of mattress or materials you tend to prefer. There is more about some of the differences between an innerspring/latex hybrid and an all latex mattress in post #13 here and the posts it links to.
Your best chance of success is to follow the steps in the tutorial post one at a time.
Phoenix