Hi nmscubagirl,
Some of the issues you are having may be connected to some of your posture issues and for some people it can be an uncomfortable adjustment to sleep in “neutral alignment” when your body has a muscle memory from your previous mattress that may be used to a “learned alignment” (see post #7 here).The best approach for the first month or so (or at least for the first couple of weeks if at all possible) is to do nothing with the layers so that your body has a chance to adjust unless there is an obvious change that would help.
There are also some suggestions in post #2 here that may be helpful because some of the issues that people have may have with a new mattress may not be because of the mattress itself and could be more connected to a pillow or mattress protector for example.
If it turns out that you do need to make some changes then post #2 here and the posts it links to have much more information that can help you with some of the detective work that may be necessary to decide on the changes that may be necessary but in most cases the most effective approach is a more detailed conversation with an experienced manufacturer so you can “talk through” your experiences and the specific symptoms you are experiencing and provide them with much more specific details that can help them to help you with some layering changes and suggestions that have the best chance of success.
If you spend a great deal of time on your stomach then it may also be worth considering using a medium layer on top with the soft underneath it to create a firmer sleeping surface because stomach sleeping is the riskiest sleeping position and stomach sleepers have a much higher risk of their hips/pelvis sinking down too far and sleeping in a swayback position which can hyperextend the spine and is a common source of lower back pain. It may also help to put a thin pillow under your hips and lower abdomen to help keep you in alignment when you sleep on your stomach.
I know that it can be frustrating but my first suggestion is time (and in the meantime you can go through some of the information I linked) and then if it becomes clear that you need to make some changes then I would have a more detailed conversation with SleepEZ and provide them with as much specific detail as you can about your sleeping experience and symptoms and try some of the different layering options they suggest.
Phoenix