Hi rceisland,
At least you have confirmed that the foundation wasn’t supporting the mattress properly because there should be little to no difference between a suitable foundation and the floor.
Post #2 here and the topper guidelines it links to have some suggestions about choosing the thickness and firmness that have the best chance of success and links to some of the better topper sources I’m aware of as well but the only way to know for certain whether a topper will be a good match for you and your mattress will be based on your actual sleeping experience so the return or exchange policy can be an important part of choosing a topper in case your “best guess” doesn’t turn out as well as you hope for.
The PLB topper would only be slightly cooler when you first go to sleep at night but won’t make a great deal of difference over the course of the night compared to a regular Talalay topper (see post #2 here).
Most of the PLB retailers I’m aware of don’t allow returns or exchanges on a topper so when you are in a trial and error mode it may be a good idea to purchase an equivalent topper from a source that allows for exchanges or returns and they will likely have a lower price than the PLB topper as well.
Phoenix