I don’t think the issue is with your ram disk, I believe the issue exists with the uEnv.txt file, it may be setting up wrong file. If you only need to grow the storage size, my suggestion will be to boot up from your nand system, simply copy over the whole file system into an sd card. Once you have done this, change the uEnv.txt file such that it points to root file system on the SD card.
This should allow you additional persistent storage. Based on your uEnv.txt file can can change the arguments.
Having said that…please remember I have not heard of any of the pros in the field doing something like this, I do it and works quiet nicely.
Hope this helps