Hi TooTall513,
I am glad that you did a fair amount of reading and that the site is helpful to you. 
While you are correct that the upper layers in a mattress (the top 3" - 6" which are the most subject to wear and tear and contribute more to the overall “feel” of a mattress) and while I understand what you are trying to achieve, for those that may be reading this thread I’d make an adjustment to your "the first line of defense " comment because in reality the primary concern should be ensuring good spinal support and proper alignment (see post #4 here for more about primary support, secondary support, and pressure relief and how they are related) because any attempt to “fix” support layers that are too soft by adding layers on top will often only be partially or temporarily successful and would be more of a “band-aid” than a solution because the top layers can still “bend into” the softer layers below them and lead to alignment issues.
I agree with you that latex is a good comfort layer choice for its temperature regulation, supporting and conforming qualities … latex is also certainly a very durable material and at your BMIs I would not be too concerned about having to replace it too soon. (I’d expect at least a 10 years of use), but you are correct that there may be also other factors involved in the durability or useful life of a mattress outside of just the material itself (see post #4 here )
Once primary support is ensured then the suitability and durability of the comfort layer will tell how well you will sleep and for how long you will sleep well. Your long-term approach of considering a bed where you can easily replace the comfort layer is very common as a solution to both fine-tune for a certain feel and also for accommodating any changes over time. (As we age we also usually prefer something a little plusher)
Based on the information you provided it is not possible to predict how well you will do with any mattress suggestion that I or anyone may have for you. Both thickness and softness are dependent and there can be numerous combinations that can work for you. Also, you can also find different categories of “multilayered systems” such as polyfoam/latex combos, all latex, pocket coil latex and you’d first need to decide on the type that would be best for you and your girlfriend. Each mattress category can also include hundreds of different mattresses with a very wide range of different designs, different “feels”, different characteristics, and different firmness levels. Every individual layer and component in a mattress (including the cover, FR barrier, any quilting material, and of course all foam layers) will affect the feel and response of every other layer and component both above and below it and the mattress “as a whole” so each mattress category will generally include some mattresses that have a design that will be a good “match” for you in terms of “comfort”, firmness, and PPP and others that use the same type of materials and components and are in the same category and may be just as durable but have a different design or firmness level that may be completely unsuitable for you to sleep on … even if it uses the same general type of materials and components.
My first suggestion would be that you do some local testing which will give you a much better sense about the types of mattresses (see this article) and general firmness levels you tend to prefer which can help you narrow down your choices regardless of whether you end up purchasing locally or online. Then some guidance from a knowledgeable and experienced retailer or manufacturer will usually result in a mattress choice that is well inside a suitable comfort/support range and will generally be “close enough” so that if any fine-tuning is necessary it would be relatively minor and involve different mattress pads, sheets, mattress protectors, or perhaps even a topper if a mattress is too firm (see post #4 here and post #10 here ).
Once you manage narrow down your research and have more specific questions then I or any of our Expert Members we’ll be happy to assist you.
Phoenix