Hi jsboogie.
In most cases, I would avoid the approach of buying a mattress to suit a topper because of the uncertainty involved if the mattress is still not supportive enough. In your case with the double flippable mattresses that you are considering adding the topper you already own will give you 5" comfort and a progressive construction and would increase your options and chances to get the right softness/firmness balance needed at your BMI and suitable in terms of thickness, firmness, and PPP on the specific mattress . Getting the right combination of mattress/topper sometimes can be almost as difficult as choosing a mattress that doesn’t need a topper in the first place. You are correct that it would best to focus on an option that is likely to be a suitable match for support without a topper and then add the topper as a “backup” strategy in case your initial choice is too firm. As far of which of the two options you listed above would work better for you I’d say that they both have a different balance of support comfort and only you can tell which one is most beneficial for you. For a Firm choice in either beds you’d probably need to add some surface plushness to it to relieve any pressure point.
While all the options you mentioned (SleepEZ, Flexus, Sedona) could work all well … only you’d be able to tell which one “achieves your sweet spot” … When you are down to final choices between “good and good” then there isn’t a formula that you can use and “best judgment” based on your testing, your conversations and questions with each retailer or manufacturer, and the specifics of your personal value equation is the most effective way to choose. I can speak only in general terms and while I’m always happy to help the forum members here with “how” to choose, to act as a "fact check, or to help them find better manufacturers or retailers that have better quality and value but “what” to choose in terms of either a mattress or the balance between comfort/pressure relief and support/alignment that may be suitable for a specific person is always up to the only person who can feel what you feel on a mattress and best left to personal testing and your own experiences or more detailed discussions with the more knowledgeable people at the retailers or manufacturers that sell a mattress you are considering. This is exactly where a good manufacturer or retailer can truly help. You seem to be on a good track with your choices but I would encourage you to always check and run everything by your internal sensors or “gut feeling”.
Placing latex on top of memory foam is a popular combination, and it assists with the temperature issues that some people are sensitive to with memory foam and reduces the feeling of being trapped in a mattress (for those who don’t like this). If you liked the overall level of support and contouring of a memory foam bed, using the latex topper on that item might work well. If it felt a bit too soft to you, you may desire the firmer version. While the latex on top would be most of your source of surface comfort (all layers of a mattress do work together), how much it “bends into” the transition memory foam layers will be determined by your firmness choice. With this type of construction, I would normally suggest a thinner layer of latex over a thinner layer of memory foam so that you can notice more of the “feel” of both materials and there would be less risk of alignment issues.
I am not sure if the mattress you describe is a custom order from SleepEZ or the dual-sided Kiss Mattress they carry. Sleep EZ is always very transparent and always disclosing the specs of their mattresses and I am sure if you ask them they’ll be happy to share it with you. Just in case for your ease of research here are their updated specs
Comfort Layers: 4" flippable. 2" of 19 ILD 100% Natural Talalay Latex, 2" of 3 lb SleepEZ Active Flo polyfoam
Support Layers: 6" 2.0 lb HD polyfoam The 2" of 3 lb SleepEZ Active Flo Foam is one of the newer generations of “high performance” polyfoams that are more breathable, faster responding, and less temperature sensitive. The 6" base layer of 2 lb polyfoam is a higher quality material than you would normally find in this category. These are high-quality materials with no “weak links” in the design and would be suitable for any weight range.
While my thoughts about suitability as it relates to your particular needs are of course generalizations, I hope that they are helpful to you.
Phoenix