Hi stack,
If you have the Glenda Plush (Twila plush) it isn’t a latex mattress at all and as you can see here it has over 6" of polyfoam in the upper layers and the only latex is a fairly meaningless .79" latex layer buried under the polyfoam.
I don’t know the specifics of the symptoms you are experiencing on the mattress, or the sleeping positions or areas of your body you are feeling them but overall I would say this could be a very risky construction for you because it would be potentially too soft. With your higher weight you may be “going through” the softer layers and feeling the firmness of some of the deeper layers (they don’t provide any information about the ILD of the layers) but with 6" of polyfoam, some fiber, and 3/4" of latex it’s not likely that any specific symptoms you are experiencing are pressure related and are more likely to be related to alignment issues.
If this is the case (the comfort layers are too thick and soft for your body type) then there really isn’t an effective solution for alignment issues because there really isn’t any way to make a mattress that is too soft any firmer by adding toppers (it would require replacing layers with firmer versions rather than adding to them). There is a little more about this in post #4 here and some of the other suggestions there may be helpful.
It may also be worthwhile removing any mattress pads or thicker mattress protectors that you may be using to see if these are interfering with the ability of the foam layers underneath to contour to your body.
If you really are experiencing pressure issues then you could try adding a 2" to 3" topper to better isolate you from the firmness of any layers that are causing pressure points but the odds of success would be low and you could be increasing the likelihood of alignment issues even more (there would be even more softer foam between you and the support layers of the mattress). At the very least I would make sure that the topper had a good return option in case it didn’t work for you so you don’t end up throwing good money after bad. Post #8 here has more about choosing a topper.
I wish I had better news but it appears to me that the basic design of the mattress may not be the best choice for you.
Phoenix