How do we set required for new & optional for existing fields - when adding basic field to content type?
You can use hook_form_alter or hook_form_FORM_ID_alter to set require for that field.

 
function my_module_form_alter(&$form, $form_state) {
  $entity = $form_state->getFormObject()->getEntity();
  if ($entity->isNew()) {
    $form['my_field']['#required'] = TRUE;
  }
}


And, not set the field 'my_field' as required field.
Not tested, but it is a way to go.