The correct save type is not FRAM 32K. Only a handful of ds games uses that save type. Since you have 2.6a, it's best to leave the save type as auto. You should try changing your game's filename without any special characters. If that doesn't work, try moving your whole no$gba folder closer to your root directory.
The corruption doesn't always happen right away. Anyways, I don't understand why is it that auto save type doesn't work for you. You could try moving your no$gba.exe to a new folder and test the auto save type again.