How to create custom node edit form
_entity_form doesn't accept a form class directly, only via a form handler. See https://www.drupal.org/docs/drupal-apis/routing-system/structure-of-routes

_entity_form: It is similar to _form, but it will provide an edit form for an entity. Entity form handlers are defined in the entity metadata
(annotation). For example, _entity_form: node.default will display the
default node form. &quot;node&quot; in node.default refers to the entity ID, and
&quot;default&quot; refers to the form handler key.