Hi Cat Lady,
Some of the theory of mattress design and how it relates to different weights and sleeping styles is linked in mattress firmness/comfort levels in post #2 here but it is only generic and not specific to any individual. Your own careful and objective personal testing following the testing guidelines along with knowing the specifics of all the layers (to make sure the mattress has no obvious weak links) will tell you much more about the suitability and quality of a mattress than trying to identify a minimum thickness which is not a particularly meaningful spec in any mattress. The only meaningful answer is “it depends” on how all the layers and components interact together and with each specific person. Every layer affects every other layer in a mattress so looking for a certain “spec” without knowing how it works in combination with all the other materials and components in a mattress is rarely meaningful.
When you are heavier, I would tend to make sure that both the support layers and the comfort layers are firmer and higher quality than the “average” because firmer materials in both (and especially in the comfort layers) will tend to be more durable and suitable and will feel softer than they would for lighter weights. In some cases and with some types of mattress it may also be worth considering a thicker mattress and post #14 here has more about this. I would also consider a strong innerspring with good quality comfort layers as well as a latex/polyfoam hybrid if they are available in your area.
You may have seenthis already but just in case … the better options and possibilities I’m aware of in the South Bend / Fort Wayne area are in post #4 here.
Phoenix