quarto render --output-dir is ignored, unless the the content to render is part of a project.
To turn a directory into a project, run create-project; hence the _quarto.yml.
Is there a different way to compose slides from various files other that using the include directive?
You need to run quarto publish gh-pages once, see here. Seems to be doing quite a lot...
You can include fontawesome through an extension https://github.com/quarto-ext/fontawesome