Hi emm0119,
Buying a topper can be somewhat tricky (just like choosing a mattress) because there are so many unknowns and variables involved (connected to the person as well as the mattress itself) that there isn’t a formula that can use “theory at a distance” to make the best choice. I would also be cautious because if the foam in the comfort layers of your mattress has softened then any topper will just follow the dips and soft spots which could be risky in terms of alignment if the foam in the upper layers of your mattress under your pelvis has also softened.
Since your biggest issue is pressure relief then a topper can help (as long as you are aware that a thicker topper also increases the risk of alignment issues) and post #2 here and some of the other posts it links to can help you make the best possible choice based on your actual experience and symptoms on the mattress.
I would choose a good quality material but the type of topper would be based on your preferences. The thinnest topper that you believe will provide you with the pressure relief you need for your shoulders so that the alignment risk is minimized would normally be the best choice. As you mentioned, toppers in most cases aren’t returnable so there is some risk involved unless you are able to test a specific topper with your mattress which is really the only way to know for certain that the topper is suitable for you outside of the guidelines I linked.
One of the ways to lower your risk is with more detailed conversations with a retailer or manufacturer that is knowledgeable and the more information you can provide them about your specific symptoms and the details of your mattress the better the odds that they can help you make the best possible choice that has higher odds of success. The other way to lower your risk is to purchase from a supplier that has a good return policy (some of the big box stores are examples) and some of the other listed options also have return policies as well that will vary in terms of the cost of returning a topper.
You can read a little more about all the factors involved in temperature regulation in post #2 here and the other posts it links to but in general gel materials will only have a temporary effect on your sleeping temperature when you are first going to sleep and once temperatures equalize then the foam will once again become an insulator. Materials and fabrics that provide good ventilation, airflow, and moisture wicking in the upper layers of your mattress (including the quilt and cover) and in your mattress protector, sheets, and bedding are the most effective way to regulate temperature over the full course of the night.
Phoenix