Hi TP,
Latex is the most durable of all the foam materials so while there will be a break in and adjustment period over the first few weeks where there may be some slight initial softening (see post #3 here) … beyond that the softening of the latex would be very gradual and isn’t something that I would be concerned about. The floor model you tested would probably have already been broken in.
I would purchase the mattress or the mattress/topper combination that your careful testing indicates is the the best “match” for you in terms of “comfort”, firmness, and PPP (Posture and alignment, Pressure relief, and your Personal preferences).
There are many latex toppers on the market that are available to purchase in any type of latex, ILD or thickness that you may need besides the PLB toppers but there is always some risk and uncertainty involved in purchasing a topper that you can’t test in combination with your mattress before a purchase because the only way to know whether a mattress/topper combination will be a good match for you will be based on your own personal experience so I would keep any topper purchase that you can’t test in person as a “backup plan” just in case the choice you make needs some additional softness and pressure relief.
If it becomes necessary … there is more information about choosing a topper in post #2 here and the topper guidelines it links to which along with a conversation with a reliable supplier (that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market) can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success. It also includes a link to a list of some of the better online sources for toppers I’m aware of as well.
Phoenix