Hi WebbyR,
Unfortunately there are too many unknowns, variables, and personal preferences involved for anyone to be able to predict which topper will be the best match for you and your mattress using specs (either yours or a mattress) or theory at a distance. I also don’t have a reference point of a mattress that has worked well for you and I also don’t know the specifics of the Optimum Brasswood. The only way to know with any certainty which topper would work best for your mattress and you would be based on your own personal testing or experience. There may be some trial and error involved so any return or exchange policies at the retailer or manufacturer you choose to buy from may also be a more important part of your purchase choices.
Having said that … and based on your feedback … it sounds like you could benefit from a softer zone in your upper body to “allow” your shoulders to sink in more deeply when you are sleeping on your side.
I don’t know of anyone that makes a custom zoned topper so you would probably need to put it together yourself.
Since you seem to prefer the feel of Dunlop latex based on your mattress choice I would guess that a reasonable starting point would be to buy a 2" Dunlop topper that was somewhere in the medium density/firmness range and then start by trying this on your mattress.
It may be OK by itself but if it seems suitable for the lower part of your body and you still need a softer shoulder zone then you can cut off a section from the head end of your topper and replace it with something softer.
I would calculate a cut line that was the distance from the top of your head to about 2" above your belly button and then add the distance between your head and the top of your mattress or headboard. This would give you a softer section under your ribcage and shoulders.
You can use an electric knife to cut your latex and then you can purchase another topper in the same thickness and in a size that will give you enough material to cut a section that is the same size as the piece you cut off from your original topper and is about 8 - 10 ILD softer to allow your shoulders to sink in more deeply.
Some of the better online sources I’m aware of for latex layers are in the component post here.
Phoenix