Hi bkemp,
I normally wouldn’t suggest the approach of choosing a mattress with the intention of adding a topper unless you can test both of them together because choosing a topper that is a good match both for the specific mattress you have (which will have a significant effect on the feel and performance of a topper) and for you can be almost as difficult as choosing a mattress in the first place and can add substantially to the uncertainty and cost of buying a “sleeping system” that works well for you (see post #2 here).
Unfortunately I can’t feel what you feel or see you sleeping on the mattress and there are too many unknowns, variables, and personal preferences involved for anyone to provide specific advice about a mattress or a mattress/topper combination that would be the best “match” for you based on specs (either yours or a mattress) or “theory at a distance” (see mattress firmness/comfort levels in post #2 here).
Having said that … now that you are in the situation you are in there is more information about primary support, secondary support, and their relationship to pressure relief in post #4 here that may be helpful. There is also more information in post #2 here about the more common “symptoms” that people may experience on a mattress that can help you with the detective work (or in some cases trial and error) that may be necessary help identify the cause of any symptoms you may be having and the options that have the highest odds of resolving them.
Based on your comments … I agree that there is a good chance that you may need some additional thickness/softness in the comfort layers to improve both the pressure relief and secondary support (the kind of support that “fills in” the recessed gaps in your sleeping profile) and if you need some additional thickness/softness in your sleeping system then post #2 here and the topper guidelines it links to can help you use your actual sleeping experience as a reference point and guideline for choosing a topper that has the best chance of success.
Before you decide on any further changes though … I would also read post #2 here about buying a mattress that is too firm because if the mattress is still breaking in or you are still adjusting to a new sleeping system then it can often be best to “do nothing” for the first few weeks to give your mattress a chance to break in and your body to catch up and adjust to a new sleeping system.
I would be very cautious about choosing a softer “base” mattress because it’s always possible to soften up a mattress that is too firm by adding a topper (or even several toppers) but it’s very difficult to “firm up” a mattress that is too soft because this would generally require replacing either a support core that was too soft or replacing comfort layers that are too thick/soft rather than adding toppers on top of the mattress (see post #4 here).
Phoenix