Hi dave3t251,
Adding a topper over lower density foam would likely increase the lifespan of the foam below it to some degree (it would take up some of the mechanical compression and stress that otherwise would be absorbed by the top layer of the mattress) but how much would depend on the relative firmness of the topper compared to the comfort layers in the mattress (how much each layer compresses) and it would also run the risk of turning the lower density soft foam into part of your support system (putting it deeper into the mattress) and compromising the overall support of the mattress.
The choice of topper is a matter of personal preference, budget, and the type of “fine tuning” you are considering to bring the mattress closer to your ideal. They will generally be fast response materials (like latex or polyfoam), slow response materials (like memory foam or some of the newer versions of slow response latex), or synthetic or natural fibers (such as a polyester or down alternative fiberbed or wool). They could be as thin as a mattress pad with minimal extra cushioning or a full fledged topper which is thicker and functions as an important part of the performance of the overall sleeping system. They can also be chosen in a wide range of thickness and firmness levels. The best use of a mattress pad or topper is to either change the surface feel and resilience of the mattress, to add a more breathable layer to encourage better temperature regulation or ventilation, or to add cushioning for additional pressure relief. It is not a great way to improve durability or support because of the risks that go with increasing the thickness of the comfort layers and putting you further away from the support layers of the mattress and because if the foams that are used in the comfort layer of your mattress are softer than the topper they will compress almost as much anyway.
In general … toppers with shredded materials or fibers or foam toppers that are relatively soft and thin will provide some localized or additional pressure relief with less risk to overall alignment because both the lighter and heavier parts of the body can sink into them relatively evenly (meaning better alignment).
So which topper would depend on the goal you are trying to achieve besides just additional durability (which by itself isn’t the best use for a topper) and of course your budget.
The first choice I would make is between fast response, slow response, or fiber.
Post #10 here may also be helpful in choosing which may work best for you. I would wait until you had slept on the mattress for a while though (past any break in or adjustment period of the first few weeks) before deciding on any fine tuning that should be added to it.
Phoenix