I'm assuming you want to unlock all characters/karts/gliders. Have you tried mk7se? mk7se is a mario kart 7 save editor.
It's a homebrew app you can use via the homebrew app launcher or be installed as a cia file via FBI, it works with the base game and with the 1.1 update installed and it's completely safe as I've used it for like 1 year+ now.
Launching MK7SE via the homebrew launcher app:
1. Insert SD card into PC via SD card adapter.
2. Google mk7se.
3. Click the first website link (it's a github page that says releases hax0kartik/Mk7se, if you see this it's the correct one)
4. Click on MK7SE.3dsx to download the file.
5. Copy the MK7SE.3dsx file you downloaded into the 3ds folder on your SD card.
6. Safely eject SD card from PC and insert it into your 3ds.
7. Turn on 3DS > open the homebrew launcher app > launch the MK7SE app.
8. Choose which region of mario kart 7 you have installed either USA or EUR.
9. D-pad to navigate up or down, A button to select, note: it'll freeze for 1-2 seconds after you select something and that's normal as it's creating files on your SD card > once done press Start button to go back to homebrew launcher and then press Home button to exit the homebrew launcher app.
10. Open up mario kart 7 and see if everything is unlocked.
Launching MK7SE app installed via FBI cia format (incase the above method doesn't work:
1. Insert SD card into PC via SD card adapter.
2. Google mk7se.
3. Click the first website link (same github website as above}.
4. Click on MK7SE.cia to download the file.
5. Copy the MK7SE.cia file you downloaded into the cia folder on your SD card.
6. Safely eject SD card from PC and insert it into your 3ds.
7. Turn on 3ds > open the FBI app > select SD > cia folder > install the MK7SE.cia file.
8. Press Home button or Start button to exit FBI app > on the 3ds home menu it'll say a new app is installed > unwrap and open the MK7SE app.
9. same as above, select region first > then select what you want to unlock > once done press Start button to exit the app.
10. Open up mario kart 7 and see if everything is unlocked.
If this works let me know.