Hi udoknowjac,
These are the types of “issues” I meant and that a mattress topper is most suitable for correcting (pain/discomfort from a mattress that is too firm but is otherwise still in good condition).
Based on your description and your past experiences and wanting “a fair bit to a lot” more pressure relief vs sleeping directly on your mattress … it seems to me that a 3" topper would be in the range you are looking for. I would also suggest that with your weight a softer version of memory foam would be a good option. This is also confirmed by your previous experience with a 3" topper which seemed to be successful.
While lower density memory foam generally feels softer to most people … this is not always the case because different versions of memory foam can have different properties and firmness levels regardless of their density.
When you are in this type of situation and you can’t test a mattress/topper combination ahead of time then the knowledge and experience of a retailer or manufacturer you are dealing with can be one of the most important parts of a successful purchase. While nobody can know for certain which topper you will do best with in “real life” in combination with a specific mattress until you actually sleep on it (because there are too many variables, unknowns, and personal preferences involved) your odds of success are much higher if you talk with someone that can accurately describe the “feel” of their toppers and how they compare to others on the market and can give you some good guidance. If you are not confident with your choice then their return or exchange policies may also be an important part of your personal value equation in case your choice turns out to be less than ideal.
So this will hopefully give you some general guidelines and the next step would be to talk with the suppliers listed in post #4 here. The ones that are members here will certainly be knowledgeable and will give you accurate information about their toppers and how they feel and perform for most people. If you give them good information about your circumstances and experiences they will also be able to give you additional guidance about which of the options they have available that they believe would have the best chance of success and then you can choose the one that you are most comfortable and confident with based on your conversations and/or that has a good return or exchange policy that will give you options if in spite of “best judgement” you end up making a choice that doesn’t work out as well as you hoped for.
Phoenix