How do I render a form from a controller?
First of all why use a _controller if all you need to do is print a form? Instead it should have _form something like this: Drupal\example\Form\DefaultForm in the routing definition.

Here is a simple guide on how to create a form in drupal 8 https://www.valuebound.com/resources/blog/step-by-step-method-to-create-a-custom-form-in-drupal-8

And if you really want to embed a form in a controller response check this out How do I embed a form in a controller response?