Hi Betsy80,
The mattress underneath a topper makes a big difference. This is why the same topper didn’t work on your old mattress because it just followed the soft spots and it did on the latex bed because the support underneath was much more even. The top few inches of the mattress/topper are what you will “feel” the most when you lie on the mattress but the layers underneath this are what keeps your back in alignment. You may not “feel” the alignment as much but it will be a big part of what you don’t feel (the back pain). the thicker the topper the more you will feel the topper itself and the less you will feel the top few inches of your mattress and the thinner the topper the more you will feel the top few inches of your mattress.
This is the difference. An “overall” sink is fine because if you sink in evenly and you are in alignment. This is the way memory foam works (you are more “in” the mattress). It’s when you sink in unevenly and your hips/pelvis sink down too far relative to the rest of your body that you have alignment issues and the back pain that can go with this.
The first step is to sleep on your new mattress for a few weeks so you know how close it is to what you want after you have gone through any initial adjustment period and the mattress has gone through its initial softening.
Once this has happened … then you can choose a topper if you need it based on the guidelines in post #8 here. There is no point in speculating on what topper you may need until you have slept on your new mattress, it has broken in, and you have gone through any initial adjustment period you will need. Then you will know for sure how your mattress will feel for the longer term and be in a much better position to decide on a topper.
Phoenix