I understand your frustration with having too many options. I was able to test product at several stores and I still got the combination wrong a couple times. Unfortunately it takes some trial and error to develop a baseline. I will echo Phoenix’s warning about focusing too much on ILD numbers but maybe the feedback will help narrow down your many choices.
You did not mention the brand of latex mattress you currently have and I am unclear what the quilting in actually made of. Regardless, I am not sure the difference in feel is as extreme as 38/45. That’s a jump.
Starting with a 9" 19/28/38 is a decent idea. You can evaluate the effect of the cover and you probably won’t have to worry about swapping the base layer, as that is a logical choice for your size.
If 19ild does not work out on top, something in the range of 22-25 seems like a likely alternative.
Be careful about going softer for the intermediate layer. I could see moving to a 28 talalay but I would start to worry about alignment issues with something softer. It’s also a concern for your partner since their BMI is higher An exception would be reverse layering where you put a firmer layer on top and a softer layer. The result is a top layer that is more supportive while the softer intermediate layer still allows for contouring. Reverse layering improved my situation but I still prefer a configuration with softer latex on top.
Generally speaking, adding/subtracting 1" of latex will have a small impact, 2" is more noticeable, and 3" is a significant change. You might add/sub 1" directly above or below your top layer to fine tune but the closer you get to the base, the less you will notice a difference.
Depending on your location, you might consider traveling to one of the vendors to do in person testing. Preferably you pick one with the greatest number of ild’s, latex types, and thicknesses available onsite. It could save you time, money, and frustration but you also never really know if something works until you try it overnight
Latex from DIYNaturalBedding has a greater price premium because it has an organic certification. Practically speaking, I’m not sure what that does for you. You might check out their Knit Ticking though. That cover has no quilting which minimizes the amount of material between you and the latex. You will also find a price premium for 100% natural talalay over blended. You can read about the pros/cons herehere in post #6 but from everything I’ve tried in person and read on the forum, the practical differences are small. Natural and Synthetic dunlop tend to be closer in price.