Send_later to Agent

Agent is thin wrapper over GenServer, so the performance should be similar.

And your example using GenServer will not be much more complex:

defmodule Counter do
  use GenServer

  def start_link(initial_value) do
    GenServer.start_link(__MODULE__, initial_value, name: __MODULE__)
  end

  def value, do: GenServer.call(__MODULE__, :get)
  def increment, do: GenServer.call(__MODULE__, :inc)

  @impl true
  def init(value), do: {:ok, value}

  @impl true
  def handle_call(:inc, state), do: {:reply, :ok, state + 1}
  def handle_call(:get, state), do: {:reply, state, state}
end

Many don’t use Agent (and prefer GenServer), and many don’t use :timer library (and prefer Process.send_after, or similar)

This is something You might learn from experience.

Just don’t use Agent if you want to handle your own messages.