Display something in the middle of post content

There is a loop variable available within your for loop which you might be able to use here (docs). This starts at 1 and increment everytime your loop runs.

Using loop.index, you could do something like this:

{# Fetch your blocks #}
{% set blocks = entry.postContent.all() %}

{# Counts how many blocks you have in total #}
{% set totalBlocks = blocks | length %}

{# Divide your totalBlocks by 2 and round to find the 50% mark in your content #}
{% set halfwayPoint = (totalBlocks / 2) | round %}

{% for block in blocks %}
    <div class="my-4">

        {# When you reach halfway, inject #}
        {% if loop.index == halfwayPoint %}
            the content you want to inject
        {% endif %}

        {% if block.type == 'text' %}
            {{ block.text }}
        {% elseif block.type == 'image' %}
            {% for image in block.image.all() %}
                <img src="{{ image.url }}" alt="{{ image.title }}" />
            {% endfor %}
        {% endif %}
    </div>
{% endfor %}