I think it is possible to nest setup_postdata() while also avoiding multiple calls to wp_reset_postdata(). This can be achieved by storing the post object that you want to revert to in a variable, and later running setup_postdata() on it.
Untested code to demonstrate:
// Inside the main loop.
// Setup outer post object.
// Store it in $outer_post_object so that we can revert to it later.
$outer_post_object = get_field('my_field');
$post = $outer_post_object; // override $post
setup_postdata($post);
// Later on, setup an inner post object
$inner_post_object = get_field('my_sub_field');
$post = $inner_post_object; // override $post
setup_postdata($post);
// Later on, when you want to revert to $outer_post_object
// Instead of wp_reset_postdata(), do this:
$post = $outer_post_object;
setup_postdata($post);
Using this technique you can have as many nested setup_postdata() as you like.
