Hi rjar,
It’s unfortunate that you weren’t able to spend more time testing the mattresses because spending 15 or 20 minutes on a mattress and carefully evaluating it for PPP (fully relaxed and using the testing guideines) can certainly make a difference but I would talk to Peter about the options you have for changing the ILD of the layers … possibly to a firmer support layer (28 ILD would be on the soft side for a support layer for many people).
If you sleep with a partner then I would also make sure that you test the mattress together because a mattress can feel and respond differently when two people sleep on it than it does with one.
Both latex and an innerspring can be a suitable and durable support layer and the choice between them would be more of a preference issue than anything else.
The “sensation” on a mattress is more subjective and it’s often the case that people “feel like” they are sinking in more or less than they are used to when they are sleeping on a mattress that uses a material that they aren’t used to sleeping on but the actual “symptoms” you experience are the guidelines I would use to decide on any changes in a mattress that may be necessary (see post #6 here)
Post #2 here has more about some of the more common “symptoms” on a mattress and some of the possible reasons that can cause them that may be helpful in combination with your discussions with Peter.
You are certainly asking the right questions but there are so many variables and preferences involved in a mattress choice that in the end you each of us is the only one that can “feel” what we feel on a mattress and everything else is just “best effort” guidance.
Phoenix