Hi psujester24.
Thanks for sharing your sleeping stats. Lightweight sleepers who also sleep on their stomachs (the flattest sleeping profile) generally benefit from thinner, firmer comfort layers. It sounds like the 1.5" comfort layer in the mattress is fairly thin, but without knowing the ILD of the foam(s), it’s hard to comment much more with regards to the mattress.
You can learn more about the qualities of memory and polyfoam in mattress comfort layers at their respective links, but in general terms, memory foam tends to compress as our bodies heat up throughout the night, creating that cradle you’re not finding especially comfortable.
Topper wise…selecting a topper mostly comes down to personal preference, and while I can’t recommend a specific topper to you, I can try to help point you in the right direction! There is more information about choosing a topper in post #2 here and the topper guidelines it links to, and there are some topper guidelines in post #8 here that you can also use.
I would say that, because you dislike the feeling of “sinking in” and because your weight and sleeping position don’t require much cradling, that you may do well to check out toppers made of natural materials (featherbeds, wool, cotton, etc.) vs foams.
There’s more detail about featherbeds in this post and more about wool toppers with some sources in post #3 here. Also look in post #8 here.
I’ve just dropped a lot of resources on you, so if any questions come up in your exploration and reading, let me know! Hopefully one or two of these resources prove useful to you as you move forward in your selection.