Tried several combinations, they either don’t work or result in site going blank. Probably the .css files can’t be enqueued that easily because you have hardcoded something somewhere. For example, I found that your bootstrap.css prioritises itself in different ways. Being the site developer, could you provide a working functions.php that enqueues all the css files under css directory? That way everyone can safely apply all the optional changes under the child-theme, as you advise.