Hi adam61,
Thanks for letting us know what you ended up buying and for your update and feedback about your initial experience as well.
It sounds like the mattress and the split layering is working very well for you.
Most importantly … congratulations on your new mattress 
Bedding isn’t one of my areas of expertise so I don’t have any specific suggestions but there is some information and a few down comforter manufacturers listed in this thread that have some good information on their site and are knowledgeable about down products in general and their own products specifically that should be able to provide you with some good information about which type of down comforter would most likely be suitable for you.
A wool or silk comforter may also be well worth considering because they are both light and good temperature regulators and can be used in a fairly wide range of temperatures. This thread may also be helpful (Budgy’s comments in particular as he is one of the most knowledgeable people I know about bedding products) along with post #3 here which includes a list of some of the manufacturers of wool comforters that I’m aware of that would also be well worth talking with.
jankdc’s link about comforters also has some good information (thanks jankdc :))
Phoenix