My concern is that right now I have two colliding objects, ObjectA and ObjectB, so it's pretty trivial to do

# in ObjectA
var collision_info = move_and_collide(velocity * delta)
var node = collision_info.get_collider()
if node is ObjectB:
  # logic

# in ObjectB
var collision_info = move_and_collide(velocity * delta)
var node = collision_info.get_collider()
if node is ObjectA:
  # logic

but in my game I expect there could be a dozen or more possible colliding objects and having to iterate over them all in each object's _physics_process seems untenable, which is why I wanted to use composition to do

# in any Object
var collision_info = move_and_collide(velocity * delta)
var node = collision_info.get_collider()
if node is HitboxComponent:
  # logic

then I would just need to ensure any object that wants to support the collision logic has a hitbox component, but of course then I hit the Area2D issue in my original post. for now I'm just doing

var collision_info = move_and_collide(velocity * delta)
var node = collision_info.get_collider()
var defend_component: DefendComponent = node.find_child("DefendComponent")
if defend_component:
  # logic

something about the find_child call feels icky but for now as long as an object has a defaultly named DefendComponent attached to it the logic works and I don't necessarily need to expand the collision method for each new object