I'm trying to use a custom model for tridents using Custom Model Data and it seems to just break. Is there any way to fix this?
Unfortunately you cannot change the model of a thrown trident with Custom Model Data, that is because it is an entity. You can only change the model of the trident in your hand.
The only way I can see this changing in the future is if there was an update to allow custom entity data, with entity data tags or something similar.
I'm gone with the wind.