Hi 2manybeds,
The PLB toppers are the softest latex they make and as Healthy Back told you your experience points to the 3" topper being too thick and soft. The 2" version is also the same softness level … only thinner … so while this would make a less risky choice than the 3" version you still may do better with something slightly firmer.
I would start with reading the topper guidelines in post #2 here and the posts it links to which may be helpful in deciding what thickness and firmness level may have the highest odds of success for you (although your own experience will always be the most effective way of knowing for sure).
Post #4 here also has some of the better sources for toppers I’m aware of but as you are mentioning when you are dealing with uncertainty because you haven’t tested or slept on a specific mattress/topper combination I would also make sure you are clear about all the specifics of their exchange or return policies which will vary from no exchanges or returns at all to much more liberal exchange or return policies.
Phoenix