Hi koulbassa, and welcome (back) to the Mattress Underground 
Sorry to hear you’ve had a less than ideal experience with your latex mattress. Thanks @CST for the input! Agreed that the comfort levels likely failed because of too-soft comfort layers for your higher BMI - appreciate the perspective from the manufacturing side.As you can see by comparing the Mattress Specifications You Need To Know to the Mattress Durability Guidelines, Latex is indeed one of the most durable mattress materials you can have- but there are a few caveats. As you know, any mattress that’s ideal for you will be based on your Stats (height, BMI, sleeping position(s) and any underlying health conditions), and your PPP (Posture & alignment, Pressure relief, and Personal preferences).
As a higher BMI sleeper, you need to find a balance between the comfort level you prefer and the support you need. If the comfort layer(s) are too soft, you can ‘sink in’ too far and bottom out on the firmer layer(s) below. This can cause aches, pains and even alignment issues. You definitely are not ‘fated’ to suffer a sagging mattress; breakdown of materials like memory foam are a drastically different issue.
Also keep in mind that different types of rubber have different ‘feels’ and properties; Talalay rubber is considered a ‘bouncier’ form of latex than Dunlop - common comparison being the feel of pound cake to angel food cake.
Your plan to salvage the good components of your mattress is a sound one, as it seems like much of the components you already have are working for you. You might want to consider replacing some of your layers with ones which more closely match your body profile, like components from Trusted Members [ of the site like APM, CST, European Sleep Works, SleepEZ, DIY Mattress, Flexus Comfort, or Flobeds, among others; if you give them your Stats, PPP, and the issues you are having with your current layers, they can assist in finding you a solution to get your sleep system back to being compatible, rather than ‘sticking it out’ or starting over. Don’t lose hope - you should be able to get back to a comfortable and supportive bed!
~ Basilio