How can I process a node form from a custom submit handler?
Typically, I found the answer 10 minutes after posting!

For anyone else, in your custom submit handler get the form object from the form state.

$formObj = $formState-&gt;getFormObject();


You can then call submitForm() on the form object and pass the form and form state variables.

$formObj-&gt;submitForm($form, $formState);


Then finally just simply trigger the save() function on the object.

$formObj-&gt;save($form, $formState);