I posted my whole workflow because it works for me without enabling extended memory, I wonder why it didn't work for you.
And I uploaded the sample project with plugin source code to Github: https://github.com/katboi01/Unity-SdmcPlugin-3DS
I'm not using devkitpro, it was compiled with Visual Studio 2015 Pro, VSI-CTR 5.0.4, ctr-sdk 11.4.0. I used the plugin from NativePluginExample as a base, and added basic file operations using functions from 'nn/fs.h' (nothig special here, other...
I made a plugin based on NativePlugin and SaveData example packages. I tested it on Citra and hardware (n3ds) and it works for me, but I could use someone else to confirm it.
In the zip there's a UnityPackage (5.6.6f2) with the plugin, an API for...
Old 3ds is impossible imo, even with most resources stripped I couldn't fit the game in under 35mb (larger than that the game doesn't start), it would have to be remade from scratch with minimal assets and scripts.
But on New 3DS it works fine...