form_state->setRedirect(to_a_new_node_page) from the submitForm function, route "does not exist" issue
I think if you want to use the setRedirect function then you might want to look at its documentation found https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Form%21FormState.php/function/FormState%3A%3AsetRedirect/8

You can then base the ff. code from FormSubmitter::redirectForm or FormSubmitterInterface::redirectForm. So your code might look something like:

$form_state-&gt;setRedirect('entity.node.canonical',
  array('node' =&gt; $mynewnode-&gt;id()),
));


You can also find out where is "entity.node.canonical" from by searching in core/modules/node and where it is declared and what it's about.

Hope everything's going great!