How can I render a specific node as the content for the maintenance page?
Use the Twig tweak module and create an overwrite template for maintenance-page.html.twig. Then copy &amp; paste stuff from your base theme, but replace {{ page.content }} from the base theme with {{ drupal_entity('node', X) }}
E.g. for stable 9 theme this would look like
&lt;header role=&quot;banner&quot;&gt;
  {% if logo %}
    &lt;a href=&quot;{{ front_page }}&quot; title=&quot;{{ 'Home'|t }}&quot; rel=&quot;home&quot;&gt;
      &lt;img src=&quot;{{ logo }}&quot; alt=&quot;{{ 'Home'|t }}&quot;/&gt;
    &lt;/a&gt;
  {% endif %}

  {% if site_name or site_slogan %}
    {% if site_name %}
      &lt;h1&gt;
        &lt;a href=&quot;{{ front_page }}&quot; title=&quot;{{ 'Home'|t }}&quot; rel=&quot;home&quot;&gt;{{ site_name }}&lt;/a&gt;
      &lt;/h1&gt;
    {% endif %}

    {% if site_slogan %}
      &lt;div&gt;{{ site_slogan }}&lt;/div&gt;
    {% endif %}
  {% endif %}
&lt;/header&gt;

&lt;main role=&quot;main&quot;&gt;
  {% if title %}
    &lt;h1&gt;{{ title }}&lt;/h1&gt;
  {% endif %}

  {{ page.highlighted }}

  {# disabled {page.content} here #}
  {{ drupal_entity('node', X) }}
&lt;/main&gt;

{% if page.sidebar_first %}
  &lt;aside role=&quot;complementary&quot;&gt;
    {{ page.sidebar_first }}
  &lt;/aside&gt;
{% endif %}

{% if page.sidebar_second %}
  &lt;aside role=&quot;complementary&quot;&gt;
    {{ page.sidebar_second }}
  &lt;/aside&gt;
{% endif %}

{% if page.footer %}
  &lt;footer role=&quot;contentinfo&quot;&gt;
    {{ page.footer }}
  &lt;/footer&gt;
{% endif %}