To use REMOVE_ITEM_BY_KEY, I believe it will only work with keys at the root level. The way I’ve approached this in my work is to dig into the array, modify the element I want, and then reset the top-level element to the new value.
This worked in testing for me: SET_KEY(appVars.data.root, "array1", WITHOUT_ITEM(appVars.data.root.array1, appVars.data.root.array1))
Essentially it drills into array1, uses WITHOUT_ITEM (a list function) to remove the item you don’t want from the list, and then sets that new list to the value of array1 using SET_KEY (an object function.)
You’ll have to figure out how to swap out the  index with your current vars, but hopefully that’s not too complicated.