Hi JBR2022,
Sorry to hear you are still having trouble finding a mattress that suits you. Part of the problem you migt be experiencing is simply the properties of memory foam – these are made to contour to the sleepers body, and therefore they, by nature, will retain the heat of the sleepers, as described in my previous reply. While a softer comfort level will allow you to ‘sink in’ more and therefor have more possibility of retaining your body heat, its’ really in the nature of memory foam to somr extent.
Just from the specs you’ve given, depending on the density of the Beautyrest which they do not disclose, the thickness of the foam layers would be ostensibly ‘better’ for not retaining heat. Serta, likewise, do not disclose composition, density or thickness of their foams, so its not possible to judge whether you personally would sleep ‘hotter’ on either mattress. The comfort lebvek of a mattress and is unique to the sleeper, and is entirely subjective. You may want to look at the trusted Members of the site who have memory foam offerings, as they do disclose materials, thickness and densities of their products, and are able to better match the desired ‘feel’ consumers are looking for by providing them a small amount of information.
Another option you might look into is using a mattress topper with a breathable material like cotton, wool or coconut coir which would allow more air circulation between you and the top layer of the mattress. Many of our members offer toppers of varying materials and firmness levels and this might provide a cooler sleeping experience. There are also products aimed at hot sleepers like EightSleep and Chillipad Which I have not personally used but you can see in this Post one of our forum member has great reviews on a hydronic cooling system, which, while not inexpensive, has solved his problem with sleeping hot.
Hope this helps,
Basilio