I'm on 5.17.4 on drupal8 and civimail is working, html editing and all. But I don't know why. CKeditor loads and it's clearly not the drupal one (minimal), but it also doesn't fully match the editor config under Display Preferences. There are no console, server or civi log errors. It does take part of the preferences into account though; I can break the editor by choosing a skin or plugin that's not available.
The pressing problem is that the toolbar does not contain the link buttons and there appears to be filtering involved, so even in the source view, adding anchor elements manually, eventually just filters them out, replacing them with the tag's content. So no way to add html links.
So where is this third config coming from? Would this be the default CKEditor ships with and is somehow used instead of the civi provided one? What are the expected lookup paths?
EDIT: I can even use
'CKEditorCustomConfig' => "adasdada" in Resources.php and it changes nothing. I also tried hacking away what I found of ACF code in the drupal plugin part of CKEditor, but that didn't have an effect either.