Hi joltman, and welcome to the Mattress Underground 
My wife and I purchased a Cal King Wynkoop St Plush from Sleep Nation in Lone Tree, Colorado back in January 2015. I’m a larger guy at 6’3" 265lb and my wife is 5’10" at 130lb and we loved the bed at first. It was very soft and we just melted into it. However, in the past couple years the bed has become much firmer than we wanted
Sorry to hear your 2015 Wynkoop is no longer suiting you. Assuming that was all latex like the current version, with your higher BMI the comfort level may have been too soft…without adequate support you can ‘bottom out’ to lower layers compressing the mattress. The current version has 2 6" layers of organic latex of unknown ILD, with a cotton and wool cover, and as you say it is flippable, but latex is one of the most durable materials you can have in a mattress and such a dramatic change in feel from very soft to too hard is unusual - as you can see in the Mattress Specifications You Need to Know compared to the Mattress Durability Guidelines
Any mattress you and your wife finally choose will be unique to you both, based on your Stats (height, BMI, sleeping position(s), and any underlying health conditions), and your PPP (Posture and alignment, Pressure relief, and Personal preferences); thanks for providing you and your wifes’ partial Stats!
For these reasons, I’m looking at DIYing the bed in several layers. A DIY approach appears to be more than 50% cheaper than the current offering at Sleep Union. But because they never told us what the layers were, I can’t really replicate what we had. With that said, we’re really wanting something that will allow us to melt into the bed and still give us support on the lower back and in the hips to ribs regions like our mattress originally did
I am curious about these layers. Do the layers tend to move over time? Like, do they slide sideways and need to be reoriented? I’ve also read that there was a particular brand latex layer that had a glue seam running down the middle of the bed. Do all brands have this?
It is not usual for layers to ‘move around’ if they are properly constructed and inside a good-fitting cover. The glue seam issue is not common, for ease of shipping sometimes latex is cut in half but there should not be a noticeable seam.
I forgot to mention that we have a nice platform now with many firm slats that allow air flow and central support. We also have two twin box frames (metal rod and wood) under the latex mattress. These have fabric covers but allow the mattress to breathe.
A DIY can be the most rewarding way to customize a mattress, allowing you and your wife to have your own comfort and support layers, if needed. It can require some trial and error drilling down to get the components right, but also allows you to interchange layers - like a plush comfort layer with firmer support layers under - until you get it right. You want a softer top layer, but still need to make sure you have adequate support to avoid any aches, pains or even misalignment. Also keep in mind Talalay latex and Dunlop latex have very different feels - Talalay is considered ‘bouncier’( think of the difference between angel food cake and pound cake). I would also make sure that you check with any manufacturer you are considering to make sure your slatted base is compatible and does not damage the mattress or void any warranty of exchange policies just in case you need to avail yourself of these.
Several of the Trusted Members of the site specialize in DIY, like DIY Mattress, DIY Natural Bedding, and Latex Mattress Factory, among others; these all have plenty of experience in assisting consumers with their DIY projects; they have been vetted for quality, great customer communication and generous exchange and return policies. You can email or call any of them and by providing them with both of your Stats and PPP they can help you begin your DIY project and assist you in finding your next great mattress that will provide many years of comfortable sleeping!
~ Basilio