Hi copenhavert,
The better options and possibilities I’m aware of in the Kansas City area are listed in post #2 here and there are a few places where you should be able to test different types, designs, and firmness levels of latex mattresses.
When you can’t test a mattress in person then the most effective approach to choosing the type and combination of layers in a mattress would be a more detailed conversation with a knowledgeable and experienced manufacturer or retailer who will know more than anyone else about their own mattresses and about “matching” different body types and sleeping styles to the mattresses and options they have available based on the “averages” of their customers that are similar to you (see mattress firmness/comfort levels in post #2 here). They can help “talk you through” the options that have the highest chance of success.
A plush topper will generally be more pressure relieving and will “feel” softer so most people that spend a significant amount of time sleeping on their side would choose a softer top layer than those who sleep mostly on their backs and stomach. Different people with different body types and sensitivities though can have a very different opinion and experience on the same mattress because the feeling of firmness and softness is also very subjective and can vary widely between different people. The only way to really know what feels firm or soft for you is based on your own personal experience because a mattress that feels too firm for one person can feel too soft for someone else.
The thickness and firmness/softness of the top layer can have “some effect” on the choice of the transition and bottom support layers in “some cases” because every layer of a mattress will have some effect on the feel and performance of every other layer and the mattress as a whole but trying to design your own mattress based on the specs of individual layers can become overwhelmingly confusing and complex and lead to “paralysis by analysis” for anyone that doesn’t have significant personal experience on a wide range of different latex mattress designs, layers, and firmness levels. It’s generally much easier and more effective to go by your own careful testing or personal experience or the guidance of a knowledgeable and experienced manufacturer than trying to design your mattress based on specs that won’t really mean anything to you anyway unless you have specific reference points of mattresses with known designs that you have tested that you can use to “translate” the specs into what they will feel like for you. Even then there are many other “specs” that can have a significant effect on the feel and performance of a mattress other than just ILD or the firmness of individual layers … see post #2 here.
I would keep the process of deciding on your layers or on a mattress as simple as possible
Phoenix