Hi atomheartmother,
Welcome to the Mattress Forum!
It seems as if the issue you have is with your platform bed, not the mattress, because as you stated the mattress feels good when placed upon the floor (which is representative of the type of surface that you’d want under your mattress). The appropriate style of foundation you’d want under your mattress is discussed in more detail in the foundation thread here.
In a nutshell, you want a surface that doesn’t flex, and unfortunately the bed that you currently have doesn’t provide that level of solidity, so I wouldn’t recommend using it with any mattress (innerspring or foam) unless you properly reinforce it. As I’m not completely familiar with the construction of your platform bed, it sounds like at a minimum you’d need to provide more reinforcement under the deck. As an example, you could choose replace all of the slats (so that the height is uniform) and reinforce these as T-slats. Even with more slats, you still might need to go with a thicker top deck than what you currently have.
You could use a flexible slat system like from IKEA as Dr.Ake recommended, but I still don’t know if your support slat system would be adequate, and a bowed slat system will slightly change the feel of the mattress. You certainly can purchase a foundation (like what you linked to – and there are other vendors listed in the foundation thread I linked to above), and it would provide an appropriate surface, but again I’m unsure of the adequacy of your slat network as it currently stands. For a mattress with a polyfoam base, you want no more than 5" between the top slats of any foundation, and 3"-4" is even better. I can’t speak to the quality of the item you linked, and the distance between their slats is 4.75", so it’s on the cusp of what I would recommend.
In the end, the mattress has no choice but to conform to what it is placed upon, and this will influence your comfort, so I would focus on addressing your support system and making this as solid as possible so that it mimics the floor, and then you should have an appropriate sleeping surface and your mattress should feel much better.
You’re welcome to post photos to the thread and I’ll do my best to comment upon what you provide.
Phoenix