@Canapin Yes! Got it all working. Here is what the plugin.rb ended up looking like. Turned out I just had a path issue where I was putting the custom version of the file.
I ended up putting it in a folder /custom_views/embed/comments.html.erb
# frozen_string_literal: true
# name: optuma_discourse_custom_embed
# about: A simple plugin to used to customise the template in the forum embeds
# version: 1.0.0
# authors: Optuma Pty Ltd
# url: https://github.com/OptDev/optuma-discourse-custom-embed
# required_version: 2.7.0
after_initialize do
::EmbedController.prepend_view_path File.expand_path("../custom_views", __FILE__)
end