Hi Rosey,
As you mentioned one of the benefits of having a separate topper is being able to replace just the topper if it softens or breaks down before the mattress (which is usually the case with the softer top layers of a sleeping system) or if your needs or preferences change down the road without having to replace the entire mattress.
There is more about the 3 most important parts of the “value” of a mattress purchase in post #13 here which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on suitability, durability, and all the other parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
While I can’t feel what you feel or see you sleeping on the mattress so I can’t speak to how any mattress will feel for someone else … latex is a very high quality and durable material so you are certainly looking at a very durable choice.
The latex in the mattress is most likely 6" and then the cover and the fire barrier would probably make up the additional inch but with the topper your mattress (or in this case your sleeping system) would be a total of 9" which is a fairly average thickness.
The thickness of a mattress or the number or thickness of any individual layers inside it is really just a side effect of the design and the design goals of a mattress and is also only one of many variables that can affect the feel and performance of a mattress relative to any particular person and by itself isn’t particularly meaningful (see post #2 here). In some cases higher weight ranges (or a higher BMI) will sometimes do better with a mattress that is thicker than lower weight ranges or a lower BMI (see post #14 here for more about the effect of thickness) but even this depends more on the specific design and combination of materials in the mattress and on how well your testing or personal experience indicates the mattress (or mattress/topper combination) “as a whole” matches your specific needs and preferences in terms of comfort, firmness, and PPP than it does on just the thickness itself.
Phoenix