Hi libertas,
I would keep in mind that memory foam in general will tend to sleep warmer for some people than other types of more breathable foam materials such as polyfoam or latex in the upper layers of a mattress.
While it’s not always possible to to track down temperature regulation issues for any particular person on a specific mattress because there are so many variables involved (including your room temperature and humidity, your sheets and bedding and bedclothes, your mattress protector or any mattress pads you are using, and where you are in the “oven to iceberg” range) and some people can sleep warmer on mattresses that most people are generally fine with … there is more about tracking down a potential cause or causes for temperature regulation issues (at least to the degree possible for a specific mattress) in post #2 here and the posts it links to that may be helpful.
Two of the options that would have the least possible effect on the “feel” of the mattress that would be worth considering are …
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A more breathable type of mattress protector that doesn’t have a semi breathable waterproof membrane and is less waterproof or water resistant would allow for more airflow and wick and store moisture and humidity more effectively can help with temperature regulation . There is more about the pros and cons of different types of mattress protectors and some examples of each of them in post #89 here.
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Sheets that are made from natural fibers such as cotton, silk, or flax linen or from different types of rayon/viscose (such as rayon from bamboo) that are also more breathable and are good at wicking and storing moisture can also help with temperature regulation as well (flax linen in particular). There is some good information about sheets in post #7 here and the other posts and sources of information it links to that may be helpful.
Phoenix