There is no way to automatically update uisettings files (window layouts, chat tabs, chat channels) for all characters at once. Logging in to each character and using load_uisettings as you have done is the correct way.
For your custom DarqUI settings you can use the Duplicate feature found on the Manage page of the DarqUI utility. Here you can choose to copy settings from one or more components over to one or more characters, all at once. Click the Duplicate button and choose "donor" and receiving characters, and components with customizations you want to copy. As with uisettings, there is no way to do this automatically.
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