Custom excerpts / post previews

I’m trying to create a custom post preview / excerpt when displaying a paginated list of posts (i.e., the standard Loop). I’m doing something like this

$context['posts'] = new PostQuery();
Timber::render('home-page.twig', $context);

and in home-page.twig

{% for post in posts %}
     ... other html like post title ...
     <p>{{ post.preview.length(25) }}</p>
{% endfor %}

{% include 'pagination.twig' with { pagination: posts.pagination({show_all: true}) } %}

I want to be able to use my own class method to create a custom excerpt based on the post content (I want to grab the first paragraph and use that).

How would I do that? It’s not clear if this is doable using the PostPreview class (https://timber.github.io/docs/reference/timber-postpreview/). I’ve tried modifying the PostQuery object but I’m not sure how to modify a PHP iterable object. I’ve tried

{{ function('the_excerpt') }}

but I run into problems trying to use a WordPress filter to modify the excerpt.

Suggestions? Thanks!