Hey there llamaduck,
For some people, purchasing a mattress without first testing it in person is almost a necessity, particularly when they prefer a softer comfort level, as many showroom models simply don’t represent the full range of available options.
Your observation that zoned support systems don’t work well for you is also not uncommon. I like a singled zoned mattress myself, but have a couple of guest room mattresses that I have tested, that have livable zoning. While zoning can be beneficial for some sleepers, others find the firmer center section uncomfortable or poorly aligned with their body type and sleeping posture.
At this point, the most productive path forward is typically not continuing to randomly test mattresses in a trial-and-error manner, but instead working with an experienced mattress professional who can carefully review your preferences, body type, and prior experiences, and help narrow the field to more appropriate constructions. Without that kind of guidance, even quality local manufacturers can end up feeling like a frustrating or futile process if the designs being shown are not well aligned with what your body responds to.
You’ve already done a substantial amount of legwork, and that experience has helped clarify several important criteria, non-zoned support, a traditional innerspring feel, and a medium pillow-top or plush comfort level without going overly soft, while avoiding latex, memory foam, and grid-style systems.
While @DLX offers some great options, what I think would be more valuable to you, is working with their very experienced team. They hear stories like yours every day of the week. Keep in mind, you dont want to sacrifice support, even robust support, in your search for a softer mattress. Support comes from the support layer, and comfort relies on the comfort layer, cover and possibly the topper. I mention topper because some folks overlook the value and place of a topper in the overall comfort and construction of a mattress. I have mentioned this many times before that some of the most luxury and premium mattresses designed and crafted in the world, create mattress systems that revolve around the use of a topper, rather than the topper being an afterthought add on. So do not overlook that factoid. Regarding a Helix Core model, someone like Ben @yawnder or Dr. Rick at Ultrabed can be a valuable source of information and guide you in the right direction.
A seasoned mattress professional’s goal is to avoid a return by nailing their client’s comfort zone out of the gate on the first go around.
With that information in hand, a knowledgeable and experienced mattress professional should be able to make the remaining decision process significantly more focused and efficient.
I know I did not address a specific mattress that might solve your issues, but the road to get there is just as important.
Maverick