The second way works. So we'll go with that.

app/controllers/registrations_controller.rb

@@ -1,17 +0,0 @@
-class RegistrationsController < Devise::RegistrationsController
-
-  before_action :one_user_registered, only: [:new, :create]
-
-  protected
-
-  def one_user_registered
-    if User.count == 1
-      if user_signed_in?
-        redirect_to root_path
-      else
-        redirect_to new_user_session_path
-      end
-    end
-  end
-
-end

config/routes.rb

@@ -5,7 +5,11 @@ Rails.application.routes.draw do
     resources :virtuals, only: [ :new, :create, :edit, :update, :destroy]
   end
 
-  devise_for :users, controllers: { registrations: "registrations" }
+ devise_for :users, :skip => [:registrations]
+  as :user do
+    get 'users/edit' => 'devise/registrations#edit', :as => 'edit_user_registration'
+    put 'users' => 'devise/registrations#update', :as => 'user_registration'
+  end
 
   # Define your application routes per the DSL in https://guides.rubyonrails.org/routing.html