Как Rails обрабатывает изменения, внесенные во вложения ActiveStorage ресурса?

Я бы выбрал первое решение, которое вы рассмотрели:

A designated controller just for managing attachments (which would omit any kind of resource specific callbacks

Это примерно то, что мы делаем в Basecamp. Вот демонстрация:

# app/models/event.rb
class Event < ApplicationRecord
  belongs_to :user
  has_many_attached :highlights
end

# app/controllers/events/highlights_controller.rb
class Events::HighlightsController < ApplicationController
  before_action :set_event, :set_highlight

  def destroy
    @highlight.purge_later
    redirect_to @event
  end

  private
    def set_event
      @event = Current.user.events.find(params[:event_id])
    end

    def set_highlight
      @highlight = @event.highlights.find(params[:id])
    end
end

# config/routes.rb
Rails.application.routes.draw do
  resources :events do
    resources :highlights, controller: "events/highlights"
  end
end

<%# app/views/events/show.html.erb %>
<% @event.highlights.each do |highlight| %>
  <%= link_to image_tag(highlight.representation(resize: "200x200>")), highlight %><br>
  <%= link_to "Delete this highlight", event_highlight_path(@event, highlight), method: :delete %>
<% end %>