Hi xlcooplx,
The good news is that even though the major brands are not great quality or value choices … you chose one that has less lower quality polyfoam in the upper layers (which is usually the weak link of a mattress) and is much firmer than average so it can be a more suitable base for a topper to add some pressure relief and comfort to your mattress. A topper will also improve the durability of the foam underneath it and has the additional advantage of being replaceable so if it wears out faster than the rest of the mattress you can just replace the topper rather than the whole mattress.
Choosing which topper though can be a matter of trial and error as each person can be very different in their needs and preferences. This thread though along with post #8 here can give you some guidelines about making the best possible choice.
The first step would be to choose the material. Your basic choices would usually boil down to memory foam, latex, polyfoam, or various synthetic or natural fibers (such as wool). I would stick with materials you have tested (or can test) and know you are comfortable with or alternatively if you are trying a material that you are not familiar with to make sure that returning it is possible with little cost (including return shipping and any fees or deductions involved).
Because it seems that you prefer faster response materials (you didn’t choose a memory foam mattress) … it would probably be better to look in the direction of polyfoam or latex both of which are fast response (unless your own preferences or testing indicates otherwise). Latex is much higher quality but more expensive and polyfoam will be lower quality and less durable but could be replaced as necessary.
Some of the better sources of toppers I’m aware of are in post #4 here. Given your body type, sleeping position, and preferences and the firmness of your mattress … I would be tending towards 2" - 3" in a slightly “firmer than average” softness level or ILD.
Hope this helps.
Phoenix