featured image with plugin

If you already have both the attachment and the post created, you can call
set_post_thumbnail( $parent_post_id, $attach_id );
https://developer.wordpress.org/reference/functions/set_post_thumbnail/

The function wp_insert_post() can take all the parameters for a post.
https://developer.wordpress.org/reference/functions/wp_insert_post/
You can insert the image first, and use the ID in the meta for _thumbnail_id of the post. But you need to update the post_parent field of the image post if you want it to be “attached”.
But that is similar to wp_insert_attachment().
https://developer.wordpress.org/reference/functions/wp_insert_attachment/