Hi PS99115,
The specs you listed for the Nature aren’t quite correct (see post #2 here) but they are close (it uses Talalay GL in the top 2" which has an ILD of 21 and the 19 ILD you listed would be “regular” blended Talalay).
If a mattress has the same layers and components (type of latex, ILD, layer thickness, and a functionally similar cover) then it would be very close to the Nature.
If there are any differences between two mattresses in any of these specifications then how close they were to each other would be much more subjective and different people could describe and experience how close they were and the differences between them in different ways. There is more about “matching” one mattress to another in post #9 here.
If you are considering a DIY mattress where you buy each layer and component individually then I would make sure you’ve read post #15 here so that your expectations of success are realistic and you are comfortable with some of the time, research, and trial and error that may be involved and any extra or unforseen costs involved if your experience indicates you need to change any of the layers.
The “best” approach would be the one you indicated where a “spirit of adventure” and fun along with a willingness to experiment and where your experiences and what you learn along the way is the most important part of the process.
Phoenix