Hi ebeck,
I would tend to avoid the temptation to assess a mattress based on how far it “feels like” your pelvis sinks down. This is often just the difference between a new mattress and what you are used to sleeping on. The key is always spinal alignment and the ability for your muscles to fully relax over the course of the night. For example if your pelvis really is sinking down too far then this would indicate a mattress where either the comfort layers are too soft or too thick or the support layers are too soft … not too firm. The goal is always the natural alignment of your body, hjow evenly the different parts of your body are sinking in, and the ability for all of your muscles to “let go”. There is more in post #6 here.
If you are past the initial break in and adjustment period and you have eliminated some of the other possibilities in post #2 here then if the retailer you purchased from has an exchange policy then it can certainly be a good idea to take advantage of it as long as the next mattress you choose improves your symptoms because there is usually only one exchange allowed.
A topper can be a great solution if more softness is needed and Pure Latex Bliss has both 2" and 3" toppers which are part of the options available for their sleeping system. There are also many other sources for latex toppers in a wide range of firmness levels and thicknesses. A mattress is only one part of your overall sleeping system and a topper can be a very useful idea because it can be used to customize and fine tune your sleeping system and can also improve the durability of the layers below it. If it wears out faster than the layers below it (a mattress will normally soften and break down from the top down) it can easily be replaced without replacing the whole mattress. There is no inherent benefit to having just a mattress itself if a mattress / topper combination is a better match for your specific needs and preferences.
Phoenix