Welcome to the Mattress Forum! :).
Wow, sounds like an exciting trip or adventure!!!
I am sure we can help, and although there are so many options you have done a good job outlining a few good options.
[quote]Here is what we are considering:
A single 6" 19 ILD Dunlop core? (will it not be supportive enough?)
Combine 2 3" toppers into a customized layer.
For example, a 14 ILD Talalay over a 20-something ILD dunlop for a plushier feel. (Is this too soft? Not supportive enough for our weight?)
Should we do a 19 ILD Dunlop over a 20-something ILD Dunlop?
Or maybe 15 ILD Dunlop over a 28 - 33 ILD Dunlop would be more similar to the Nest bedding version? (even tho theirs have nearly 6" of soft on top and 6" of firm on bottom")[/quote]
I can definitely give you a good recommendation, and for a little background, I think using two 3" layers is important, a 4" and 2" is also feasible, but buying 4" pieces is harder to purchase. See this overview link about putting the layers together. We are using a differential construction explained in more detail here.
Also, please see our trusted member list as several of our members can sell you the latex foam in raw topper form.
Another thing to keep in mind is that the great beds at Nest Bedding, are fully finished closed, with layers laminated together so it firms up the mattress this way. Using two 3" layers covered by yourselves will be much softer feeling than when they are used in a finished mattress. One other thing is a 14 ild Dunlop does not feel the same as a 14ild Talalay. You can read more about this in this latex article.
The top 3" I would recommend you use a 19 or 24ILD Talalay. Don’t think the 24 is too firm as it will feel much softer as a separate layer. The bottom 3" can be either Dunlop or Talalay and I would use a 32 ILD. I do not think you should go to a 14ILD Talalay as its too soft and you are only working with 6" to do the work of the whole mattress.
If for some reason, maybe for convenience sake, you needed to get ONE 6" core and the layers were too complicated to work with, then I would say 28ILD Talalay.
Will this exactly match the “soft” version at Nest…no not exactly as this is really impossible to do, but it will give you some guidance to get very close.
Good luck on your trip, and please don’t hesitate to message back if you have any other questions. I am looking forward to hearing how it all works out.