Hi sleepless in wi,
You are in the same boat as many people trying to balance the needs of spinal alignment with pressure relief which by their very nature are conflicting needs. Support requires firmness while pressure relief requires softness.
The good news though is that the softness part relates more to the upper layers while the firmness relates to the deeper parts of the mattress so while each affects the other, they can also be adjusted independently of the other. The trick with this is to pay attention to the firmness of the support layers and to BOTH the thickness and softness of the comfort layers. Thickness especially is the “neglected” factor. The closer to the support layer you are … the more it can hold up your heavier parts and help with best alignment. The goal is to use the firmest and thinnest comfort layer that adequately relieves pressure on your shoulders. In your case this will likely be in the range of 3" although if 2" worked it would be even better.
An example of this is when you were sleeping on the floor on a pad. This would have been good for support (your pelvic area couldn’t sink down too far) but not to good for shoulders (couldn’t sink in far enough to spread the pressure out to the torso without bottoming out).
So the goal in mattress testing is to test out various mattresses where the thickness and softness of the comfort layers is known so that it is easier to make adjustments (softer/firmer or thicker/thinner) from a known set of specs. For example it would help to know the thickness of the orthogel layer of the mattress you tried and also the support layer underneath it. My tendency (as you also suggested) would be to start with a “firmer” support core (36 ILD or so) and then experiment with various comfort layers over top of this. Without knowing the thickness of a comfort layer you’ve tried it’s difficult to know which direction to go. What you are looking for with each layer you test is first of all pressure relief (laying on your side completely relaxed to sense any pressure points on your shoulders) and if the comfort layer is suitable for pressure relief then to test for alignment on your back (completely relaxed and sensing for any tendency to keep the muscles in your lower back tense). You want the mattress to keep you in alignment rather than your muscles.
Another solution that gives you more flexibility in the choice of comfort layers would be to test zoned support cores where the middle zone is firmer which means that the upper layers can be either slightly thicker or softer without sacrificing support and alignment. Zoned comfort layers can also help with this although they are less common.
So a thinner comfort layer is better … as long as it’s not too thin for optimal pressure relief on your side.
A firmer comfort layer is better … as long as it’s not so firm that even with the optimal thickness you don’t sink into it deeply enough with your shoulders.
A zoned core will allow you to go a little thicker or softer than a core without zoning.
A “standard” starting point would be to try a latex core in an ILD range of about 36 (firm) with a comfort layer in the range of 3" and 19 ILD (your weight is lower so softer 19 may be better than slightly firmer 24 ILD). If that works then trying 2" for pressure relief would be the next step to see if this works. If this is too thin then slightly softening the support core or using a middle transition layer which is slightly softer can “help” the comfort layer and still be firm enough for good alignment. Testing other materials in the same general thickness would also help.
This suggestion isn’t “exact” and it’s more important to have a beginning reference point with specific feedback than it is what that “beginning point” is. Each person has a different weight distribution and even a different natural curvature and flexibility of the spine so actual testing and feedback based on your own experience is the best way to know whether to go thinner/thicker and/or firmer/softer in the various layers.
One further suggestion is that local factory direct manufacturers or sleep shops that carry smaller brands and are more knowledgeable and transparent about the layers in their mattresses (and why a particular layering scheme may work better for you) will almost always give you better advice and help that leads to better choices than chain stores or other outlets that focus more on national brands with so called “proprietary” layering.
If you let me know which city you live in I’ll be happy to take a look if I know of any better options that are near you.