How to render initial posts on page load in a Gutenberg block using the Interactivity API?

You could consider running actions.getPosts on mount via data-wp-init like:

<div … data-wp-init="actions.getPosts">

As per the documentation:

This directive runs a callback only when the node is created.

You could also consider server-rendering the initial set of posts, like:

<div data-wp-watch="callbacks.renderContent">
  <?php
  $query = new WP_Query(
    array(
      …
    )
  );
  ?>
  <?php while( $query->have_posts() ) : ?>
    …