Hi PhilMc,
While there are too many unknowns, variables, and personal preferences involved for me to make any specific suggestions (see mattress firmness/comfort levels in post #2 here) … I may be able to provide some more general information or suggestions that may be helpful (I’m not sure which specific posts you’ve already read).
I would be very cautious about believing the density of this topper and the “5.3” is just a number that is part of the name of the product or a “rating” but not a density number (see post #12 here).
There is more in post #2 here and the posts it links to about the different “symptoms” that people may experience on a mattress and some of the possible causes for them that may help you decide on the type of fine tuning or changes in your sleeping system that may be helpful.
There are also some suggestions in post #2 here that that may also be helpful if your mattress is too firm.
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here. that may also be helpful in deciding whether you need firmer or softer support layers or firmer or softer comfort layers.
The most common (but not the only) cause of lower back pain is generally in the deeper support layers and it may be helpful to re-arrange your layers so that you have the medium on the bottom, the firm in the middle, and then the soft on top which can improve the support/alignment under the heavier parts of your body (hips/pelvis). The difference this makes in your “symptoms” compared to the layering you are using now can also provide a pointer to any other changes that may be helpful. I would also focus on small incremental changes so that you can use what you “learn” from each change to decide on any additional chances that may be necessary.
The most reliable source of guidance though with any fine tuning adjustments will always be a more detailed conversation with the manufacturer or retailer that sells it because they will know more about the options they have available and “matching” their mattresses and layering combinations to different body types and sleeping styles than anyone else so the first step I would take is a more detailed conversation with Neal at Spindle.
I would first start with re-arranging the layers that you have so that you can “learn” as much as possible about the types of changes that may be helpful or necessary before buying toppers or exchanging layers. Again … one small incremental change at a time and paying careful attention to the changes it makes in your symptoms is generally much more effective approach than a more “random” approach that is relying more on just trial and error.
I would prioritize alignment over pressure relief and I would start with just the mattress layers (without any toppers) and then when you are at the point where you are confident you only need some additional pressure relief and your alignment is fine and if there aren’t any softer top layers available then I would use the the information in post #2 here and the topper guidelines it links to as a guideline to decide on the type, thickness, and firmness of a topper that will have the best chance of success.
Phoenix