Render Custom Gutenberg Template

Hi, I’m building a theme and need help with the following:

I have a custom post type that’s publicly queryable, so I have pages like this:
example.com/post-type/my-post

I need automatically generated sub-pages on which I can show a form, like this:
example.com/post-type/my-post/form

This is partially working now with a rewrite rule:

add_rewrite_rule(
	'^post-type/([^/]+)/form/?$',
	'index.php?post_type=post-type&name=$matches[1]&form=1',
	'top'
);

And I can intercept the request in the template_redirect hook:

if(empty(get_query_var("form"))) {
	return;
}

$postId = get_queried_object_id();

(I also registered the query vars using the query_vars filter)

Template:

I created a custom template in Gutenberg and now I have the ID and the template available in my code:

$templateId = 26;
$templateName = "wp-custom-template-sample-form";

How can I render this template / tell WordPress to do so?

I tried the template_include hook, but it needs an absolute path to a php file which I don’t have, since it’s a Gutenberg template stored in the database.

Or do you have any other ideas on how to build this? Maybe the whole approach is wrong.