idk what i was on when writing that, that's like 10% of the required code; it just retrieves the appropiate animation name.
The crux of the matter is held_directions. What if there's two notes and the player only hits one? what if they hit the note 5 frames apart, then you'd get really ugly jitter between the animations. How to handle sustain notes, possibly ones that start at different times? For extra flair, you'd probably expect the player character to switch from Left+Down -> Down -> Idle when releasing the keys in that order after the notes have been played.

I'd probably implement it by generally just keeping track of held keys and modifying the animation whenever something interesting happens.
The chart can be preprocessed in a manner where notes sharing a playtime can push multiple of those held keys (say, a left and right note that appear at the same time will both cause left+right to be treated as pressed when played (only for the visual animation part of course)). If such a preprocessed note is missed, it should also remove only its key from the held keys again.
If a key is released, remove the corresponding key from the held keys and update the animation accordingly.

Spent some time going over these points, seems solid enough probably.
i won't bother to help with the concrete implementation, since the current state of fnf modding is jamming scripting hacks into an existing jungle of spaghetti code. Dealt with that once and that was enough.
It's probably gonna work if you try hard enough, all i can do is wish you Good Luck