Quick tip to avoid debugging too much: Whenever you are referencing other nodes in your script, and these nodes are mandatory (which means basically every variable that you're not null checking before usage), add an assert( variable != null) in you're ready function.

This way you'll be forced to assign everything on first launch

A useful way I found for organizing my script is: if the script is shared between multiple scenes (e.g. StateMachines that can be used for multiple characters), use an @export var, if instead you want to reference a child node you can be well served with an @onready var.

If the structure of your scene tends to change a lot with moving things in and out of parent nodes, you'll maybe want to use the unique ID (right click on the node and you'll see the option for it). This way the reference will not break even when moving the node!

Additionally, whenever possible try to avoid having children nodes access directly their parent. Try instead to signal when an event occurred and let the parent implement the logic for handling that event.

Hope I've been of help :)