I’m sorry to hear that your mattress isn’t working out as well as you hoped. It’s certainly not because you are doing anything “wrong” … just that you will need to figure out what to do to correct the issues you are having. It’s not unusual at all that someone doesn’t get things “right” right away.
There are 3 parts to PPP which of course are Posture and alignment, Pressure relief,and Personal preferences. The first two P’s are more about what you need and are more objective and the last P is more about what you prefer and is more subjective.
If your mattress isn’t a good match for you in terms of either pressure relief or alignment then you will usually experience some type of symptoms over the course of the night or when you wake up in the morning. These “symptoms” are the clues that can provide some insights about whether you need to make changes to a mattress.
The other parts of your experience such as how far it “feels like” you are sinking in or other more subjective perceptions that don’t produce actual symptoms are often just a matter of getting used to the feel and performance of a different type of material that you aren’t used to sleeping on. It’s not unusual at all for someone that isn’t used to latex to feel like their hips are sinking in further than they are used for example because latex is a very point elastic material which conforms differently to the shape of the body than other materials. It’s always a matter of how evenly you are sinking in rather than how far you are sinking in. Most of these types of perceptions are because your memory is using your previous mattress as a reference point.
While it may have “felt like” you were sleeping on a cheap foam mattress … this certainly isn’t the case but of course each person can have very different perceptions than someone else because each person’s experience is partly “objective” and partly "subjective. This would be similar to one person describing the taste of a food in one way and someone else describing it very differently.
So the first thing I would have suggested is to decide whether your experience on your first mattress was more a matter of how it “feels” which is subjective (and it should feel very similar to the mattress you tested in the store) or whether you were experiencing any specific symptoms which are more objective.
If you were experiencing specific symptoms … then the next step would be to decide on the underlying reasons behind them and whether they were related to the comfort layers or the support core or both. Post #2 here and the posts it links to have more information about some of the different symptoms that people may experience that may give you some insights into the reasons behind them. If the only issue was that you were sinking in too far then the three things that could change this would be reducing the thickness of the comfort layer, increasing the ILD of the comfort layer, or increasing the firmness of the support layer. Each of these would also have a different effect on the feel and performance of the mattress outside of changing how much you are sinking in.
Since you changed your mattress to a firmer core … you now have some specific symptoms (sore shoulder blades) which may also be something that is part of the adjustment period or may require additional changes to your mattress. If your symptoms are coming from back sleeping then it could mean that you need a firmer comfort layer if the soft latex in the comfort layer is contributing to sleeping in a “hunched” position because 3" of soft latex wouldn’t normally produce pressure points in your shoulder blades if you are a back sleeper. Most people also don’t feel like they are “sinking in” to soft latex as much as you do but given your experience it may also be worth considering a firmer comfort layer. Of course increasing the firmness of the comfort layer could also make it a little too firm for your side sleeping but you could also correct this with a relatively thin topper that added some additional softness and thickness but not so much that it risks alignment.
I would also check to make sure that any sheets, mattress protector, mattress pads, or the foundation you are using isn’t contributing to your experience.
It may take some detective work and some further conversations with the manufacturer to help you decide what changes you could make that will help you but assuming that you like the “feel” of latex and are OK with sinking in more in some parts of your body than you may be used to (again assuming that it’s not an indication that you are out of alignment and you are experiencing any symptoms) then you should be able to find the combination that works best for you within any limitations of the design and the options they have available.
If I had to guess though … I would guess that a slightly firmer comfort layer may solve the issues of how far you are sinking in although “theory at a distance” is always just a guess. It may also be helpful to go back to the showroom now that you have the reference point of your sleeping experience and see if one of the other combinations appears to solve the issues you are having.
PS: I read your second post after I finished writing this one and the mattress you linked uses a 6" Dunlop core and then an additional 2" of softer Dunlop latex on top of this (which would be firmer than your current top layer both because it’s Dunlop which feels firmer to most people than Talalay in a similar ILD and also because it’s thinner and will “allow” more of the firmness of the support core to “come through”).
If you can find out the density of the Suite Sleep Dunlop layers then it would be helpful as a reference point but it does seem to point to the need for either a firmer or thinner comfort layer than the 3" of soft Talalay you currently have.