Hi JaxieJ,
Your symptoms “sound like” they are connected to excess pressure in the hip area in which case a topper could certainly help but it’s not really possible to know for certain with more complex health issues because there could also be other causes connected to your injury that could lead to hip pain as well. The only way to know for certain whether a topper would be an improvement would be based on your actual sleeping experience.
There are some suggestions in post #2 here that may be helpful before you try a topper … especially looking at your mattress protector or any mattress pad that you are using to see if it is affecting the amount of heat that is reaching the memory foam which can interfere with it’s ability to soften with temperature, to make sure that you are using a solid foundation that isn’t sagging, and to make sure that the temperature in your room isn’t so cold that it keeps the memory foam firmer. I would also try and sleep with a pillow between your knees which can help keep your hips in better alignment just in case what you are experiencing is connected to alignment issues rather than pressure issues.
If none of these seem to help your symptoms I think a topper would be well worth trying and post #2 here and the topper guidelines it links to can help you use your actual sleeping experience as a reference point for choosing a topper that would have a reasonable chance of success.
Since you appear to prefer memory foam I would probably stick with it as a topper material and something in the 2" range or less would probably have a lower risk of being too thick and soft which could increase the risk of alignment issues that can come from adding a topper that is too thick.
Phoenix