Hi hmorneau,
I would start with some of the suggestions in post #2 here to eliminate some of the other variables that could be contributing to the firmness of the mattress and make sure you have given the mattress enough time.
If you do decide to exchange the mattress for the Beautiful then I would make sure that you test it very carefully and objectively using the testing guidelines linked in post #1 here because it will probably be your only exchange and you will have no other options available if it doesn’t work out for you or is too soft.
A topper has the advantage of giving you a few more options because even though it can be difficult to choose the best topper/mattress combination without testing them together in person … you can also make further changes to your mattress if you get it wrong just by changing the topper instead of having to take the loss of replacing your entire mattress. As you mentioned you can also replace just the topper if it softens faster than the other layers of your sleeping system (a mattress tends to soften from the top down). If you do decide to go in that direction then post #8 here has some topper guidelines that may be helpful.
You’re right that lighter weights generally need softer layers and there is also an adjustment period with any new mattress or sleeping surface.
Pure Latex Bliss has a very soft 2" and 3" topper (14 or 15 ILD) that may be worth testing in the store on top of the Nature to see how that compares to your mattress and the Beautiful.
Stomach side sleepers are also somewhat of a challenge because stomach sleeping needs thinner and firmer comfort layers to prevent the spine from hyperextending and sleeping in a swayback position while side sleepers need thicker softer comfort layers because of the more “curvy” body profile and the need for more pressure relief on your side. The key is “just barely enough” in terms of thickness and softness for reasonable pressure relief on your side and no more so that the risk of alignment issues and back pain from sleeping on your stomach is less.
Phoenix