Hi Shopper,
You have a much smaller than average body type which means that you won’t sink into the mattress as easily and foams that feel soft for others will feel firmer to you and lighter weight ranges will often do better with softer latex layers than people that are in higher weight ranges.
You may have read this already and it seems like you have already tried some of them but there are some suggestions in post #2 here that may be helpful.
If I was in your shoes I would consider a latex topper in some of the lower ILD ranges that are available (in the mid teens) which may give you the additional softness and pressure relief that you appear to need. An “average” choice would be in the range of 2" or so which would give you “a little to a fair bit” of additional softness. Thicker would be softer yet but if you are adding a topper for both sides of the mattress a thicker topper may be riskier for your husband in terms of alignment.
There is also more information about zoning in post #11 here and while zoning can be an effective solution in more challenging circumstances … it can also be tricky to get the zoning right so that it’s suitable for your body type and sleeping positions and there really isn’t any way to predict whether a particular zoning system will work for someone until they’ve tried it in person.
The better sources for knit covers that I’m aware of are listed in post #4 here (including SleepEz who carries one) and post #4 here includes the ones that have a thinner stretch knit cover that may help you narrow down your options a little more.
The first step in choosing a “non toxic” mattress is deciding on the materials that meet your personal “safety” criteria so you can answer the question “how safe is safe enough for me?” on a personal basis because different people can have very different answers and be comfortable with materials that they consider to be “safe” that other people may not be comfortable with having in their mattress at all. Once you have decided on the materials you are comfortable with having in your mattress then finding a mattress that includes those materials becomes much easier. There is more information about choosing a “safe” mattress and some of the differences between “safe”, organic, natural, and green materials in post #2 here and the posts it links to that should be helpful.
I would start with what you already have though and see if there is a solution that would work for you before deciding to start all over again because you may already be most of the way there and only need a softer topper.
Phoenix