I have this problem before, and this is my solution, you can reference it. I tested your script too, and it doesn't allow overrides. Maybe you can change it with dictionary method, but I think it good for beginner.
var direction := Vector2.ZERO
var move_speed = Vector2()
if Input.is_action_pressed("move_right"):
direction.x = 1.0
elif Input.is_action_pressed("move_left"):
direction.x = -1.0
if direction.x != 0:
move_speed.x = SPEED
direction.y = 0
elif Input.is_action_pressed("move_up"):
direction.y = -1.0
elif Input.is_action_pressed("move_down"):
direction.y = 1.0
if direction.y != 0:
move_speed.y = SPEED
direction.x = 0
move_and_slide(SPEED * direction)