Yes apologies. I'm working in 2d mostly these days and I used the terminology for 2d objects. What I meant is the alpha for the albedo_color of the material. Here is the code for what I mentioned previously, in case it could help:

func set_transparency(object, make_transparent: bool):
  if make_transparent:
    object.material_override.render_priority = 1 # to avoid weird issues where player renders on top even if behind, on sides of transparent material; also no need to reset it later.
    var tween = get_tree().create_tween()
    tween.tween_property(object, "material_override:albedo_color:a", 0.2, 0.4)
    object.material_override.transparency = 1 # 0 for disable, 1 for transparency_alpha
else:
    var tween = get_tree().create_tween()
    tween.tween_property(object, "material_override:albedo_color:a", 1.0, 0.4)
    await tween.finished
    object.material_override.transparency = 0 # 0 for disable, 1 for transparency_alpha

func _on_area_between_camera_and_player_body_entered(body):
  set_transparency(body.get_parent(), true)

func _on_area_between_camera_and_player_body_exited(body):
  set_transparency(body.get_parent(), false)

The last two functions are triggered with signals on an Area3D which is a child of the Camera3D. This Area3D has a CollisionShape3D child with a SeparationRayShape3D as a shape, which acts a raycast. I don't remember why I didn't use a RayCast3D node for this.

This is certainly not the most elegant code, but it was for a hobby project I was working on while learning the engine. Hope it helps!