Hi hotpancakes,
I’m sorry to hear you’re not sleeping so well 
. Tossing an d turning can be related to your body requiring you to reposition during the night. There are of course many other environmental reasons that can cause this (ambient room noise, light, mental stress, etc.), but let’s focus upon potential mattress issues. It could be, that as you suspect, you may need a bit of additional surface comfort. There is some information about the many different symptoms that people may experience on a mattress and some of the most common causes behind them in post #2 here that may be helpful. It is common for someone sleeping upon their side to need a bit of extra surface plushness. You’d also want to revaluate the thickness of your pillow to make sure that it is allowing for a more “neutral” neck alignment.
While it’s not possible to make specific suggestions because of all the many variables involved that are unique to each person and your own experience is the only way to know for certain whether any mattress/topper combination will be a good “match” for you in terms of PPP, there is quite a bit of good information about choosing a topper in post #2 here and the topper guidelines and sources it links to.
Because of the uncertainty involved with purchasing a topper where you can’t test the combination in person … a good exchange/return policy can also reduce the risk of an online topper purchase so I would also make sure you are comfortable with the options you have available after a purchase to exchange or return the topper and any costs involved just in case a topper you choose also doesn’t turn out as well as you hoped for.
Regarding cooling memory foam claims, you can read more about phase change materials in post #9 here and at the end of post #4 here) and you can read more about the various different types of gel foams in post #2 here. In very general terms … gel and/or other thermal conductive or phase change materials can have “some effect” on the sleeping temperature of a particular material but how much of an effect they will have and how long the effect will last will depend on the specific formulation of the material and on the “combined effect” of all the other materials and components of the sleeping system including your sheets, mattress protector, and bedding. Many thermal conductive or phase change materials tend to have a more temporary effect when you first go to sleep at night or over the first part of the night than they will over the entire course of the night.
Overall, it’s not really possible to quantify the sleeping temperature of a mattress for any particular person with any real accuracy because there are so many variables involved including the type of mattress protector and the sheets and bedding that you use (which in many cases can have just as significant an effect on sleeping temperature as the type of foam in a mattress) and on where you are in the “oven to iceberg” range and because there is no standardized testing for temperature regulation with different combinations of materials … there is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here that can help you choose the types of materials and components that are most likely to keep you in a comfortable temperature range.
In very general terms … the materials, layers, and components of a sleeping system that are closer to your skin will have a bigger effect on airflow, moisture transport, and temperature regulation than materials, layers, and components that are further away from your skin and softer mattresses or foam toppers will tend to be more “insulating” and for some people can sleep warmer than firmer versions of the same material.
All foams insulate to some degree, so the more you are “in” a mattress the less of your surface area is exposed to heat exchange, versus sleeping more “on” a mattress, where there is a lower “insulating” impact. Talalay latex will tend to be the most breathable of all foams, followed by Dunlop latex, polyfoam and then memory foam. Polyfoam and memory foam pourers are constantly working on newer versions of theirs foams to be more open-celled and breathable.
Phoenix