We found the official Discourse BBCode plugin more difficult to modify (because of our low discourse plugin developer skills) than this one:
Awesome BBCodes Plugin
Awesome BBCodes Plugin: multi-paragraph, nestable, with auto-complete
The " Awesome BBCodes Plugin" was quite easy to fork, clone and hack / modify; much easier than the “Discourse BBCode” plugin.
My suggestion is to anyone ( @TheDarkWizard ) looking to create custom BBCODE is to start with the " Awesome BBCodes Plugin" and hack away from there.