I don’t think one would be able to make the 8M device behave like a 2M one. It’s not about partitioning, it’s more about the fundamental hardware block size used when the device erases and writes new data. That means the routines that do writing have larger buffers.
I think the most fruitful approach is to use the SYS 1.60 which supports the 8M device and then adjust the apps to be compatible with this. The calls made to the SYS are pretty much the same with the main differences being in the memory allocation, so it should be relatively straightforward. This is also evidenced by the gabonator app which largely works with 1.6 except for storing data. I did start to look a this as a family member has a 8M model whereas mine is 2M, but have not had much time to pursue this further recently.