This experimental console application can extract files from NDS ROMs. The best part is that it's made with .Net Core and is by extension cross-platform (without needing any frameworks to be installed). It currently cannot rebuild NDS ROMs, but ndstool should be able to accept the files this tool creates.
ToolkitConsoleCore.exe <ROM> <OutputDirectory>
ToolkitConsoleCore <ROM> <OutputDirectory>
Since I don't have a Mac, I can't say what the OSX usage is.
If anyone wants builds for other OS's, I can theoretically provide them if your OS is listed here.
Note: The file size being over 100MB is the result of 8 platform-specific builds being uploaded. After you click Download, just choose the one you want and the it will only be 20MB max.