Hi jakofin.
You are welcome!
While it’s not possible to “diagnose” mattress comfort issues on a forum with any certainty because they can be very complex and there are too many unique unknowns, variables, and complexities involved that can affect how each person sleeps on a mattress in terms of “comfort”, firmness, and PPP or any “symptoms” they experience…adding the extra layer with the plush on top would increase the surface plushness and the total of 6" would be thick enough to contour around your body forming a comfort cradle which will better prevent you from “falling” all the way through and feeling the support layer underneath. The 3 zoned inner spring unit would still offer a good deep support but again it depends on your specifics, BMI, sleeping positions needs and preferences and I cannot tell if this would be supportive enough for you.
It is impossible for me to assess through an online forum if the transition layer would be “too much” for you. Many people have similar designs to the one you are now considering and this works very well for them but this does not mean that it would work well for you as again this is too subjective … you are the only one that can feel what you feel on a mattress and there are too many variables that are unique to you … for anyone to predict with certainty if the added transition layer would be “too much” for you.
Latex Mattress Factory is one of our Trusted Members here which means that I think very highly of them and that I believe that they compete well with best in the industry in terms of their quality, value, service, knowledge, and transparency and I wouldn’t hesitate to recommend them for your consideration. You are correct that neither they, I, nor anyone else can predict for sure if this would be supportive enough and work for you, but you are moving in the right direction by replacing the soft with the medium and/or by adding another layer to prevent you from feeling the innersprings core under your hips. It is also worth mentioning that Latex Mattress Factory caries both 2" or 3" toppers that come with a 30 days trial period and free shipping and if the thickness or ILD of the layer is not quite right then you would have good options to fine tune it and make it work.
A stretchy mattress protector will allow for more initial conformation and for more point elasticity and may help with the wrinkles that your mattress cover developed. Mattress protectors using a polyurethane film are popular choices for use with “traditional” and component-style mattresses. Protectors using thinner polyurethane films will be more breathable. As far as for sources, there are hundreds…you may wish to read through some of the popular types of mattress protector options that are listed in post #89 here. Most of our Trusted Members here offer good quality mattress protectors. I do not keep a list of all the accessories they carry but most of them would have an option or two that fits your criteria.
Poenix