I'm still looking for a Mario Kart 8 save file with everything unlocked. I've managed to grind myself a little and I've unlocked all characters and all cups, but only around 550 coins worth of customization, so a lot of customization is still missing. Still, it's better than starting from scratch.
You can unlock most of the remaining customization by playing pretty much any mode, even multiplayer. Just collect a lot of coins. If someone would upload a better save file that'd be nice. You may start from this file and play on it if you wish. If you play multiplayer it shouldn't take too long.
I'm not 100% sure if this will work since I deleted some files in order to make it small enough so I could upload it here. If it doesn't work I might try to find some alternate solution, such as either uploading several zip files, or uploading it through some other source. I think it might still work though, I suspect those larger files might have been replays or something.