Hi only1emme,
I’d be aware that a topper will change the feel and performance of the mattress … not just the temperature.
Some of the things that can affect the sleeping temperature of a mattress are listed in post #4 here and it may be worth considering these before adding a topper.
There are also mattress pads (thicker than a protector but thinner than a topper) which are talked about in post #10 here that may help as well.
If you decide to go in the direction of a topper then post #4 here has some of the better sources I’m aware of.
Phoenix