Multiple custom field sorting on wordpress archive page - wordpress

Question

I have been through several topics on sorting taxonomies and custom fields, including some that were promising. However, I am still not constructing my query args correctly it seems.
I am "fixing" someone else's code in a custom theme that is used in a multisite configuration, but I am able to override/filter the query on a per-site basis.
The query is called via the theme with the following:
return apply_filters( 'theme_query_args_filter', $args);
I have custom fields in the "role" taxonomy that include:
last_name
first_name
Pretty common I think.
However, when the args are executed, the following filter args are sorting only by last name (key part is the else clause):
function my_args_filter( $args ) {
if (is_home() && !is_search()) {
$args['meta_key'] = 'event_date';
$args['orderby'] = 'event_date';
$args['order'] = 'DESC';
}
else {
$tax = $args['taxonomy'];
$theterm = $args['term'];
$args = array (
'taxonomy' => $tax,
'term' => $theterm,
'meta_key' => 'last_name',
'orderby' => 'meta_value',
'order' => 'ASC'
);
}
add_filter( 'theme_query_args_filter', 'my_args_filter' );
I've tried to modify the orderby as indicated in https://make.wordpress.org/core/2014/08/29/a-more-powerful-order-by-in-wordpress-4-0/ to use an array to do a multisort, but I'm hitting my head up against the wall. I think the problem is that the code is written using a mixture of old ways of doing things and new ways.
Any advice is appreciated. According to the example on in the docs above, I SHOULD be able to pass in multiple meta key/value/orders via an array, but I'm just not getting it.
Thanks for any leads you might have. (long-time listener, first-time caller)
(I also looked at https://wordpress.stackexchange.com/questions/109849/order-by-desc-asc-in-custom-wp-query but I couldn't extrapolate that example to this one either)

Answer 1

Figured it out on my most recent try. Hat tip to this solution https://wordpress.stackexchange.com/questions/249881/multiple-custom-fields-for-orderby-in-wp-query by Nath. if there is a more elegant way to do it, please let me know.
$tax = $args['taxonomy'];
$theterm = $args['term'];
$paged = $args['paged'];
$args = array (
'taxonomy' => $tax,
'term' => $theterm,
'paged' => $paged,
'posts_per_page' => '10',
'meta_query' => array(
array(
'relation' => 'AND',
'last_name' =>
array(
'key' => 'last_name',
'compare' => 'EXISTS',
),
'first_name' =>
array(
'key' => 'first_name',
'compare' => 'EXISTS',
),
),
),
'orderby' => array (
'last_name' => 'ASC',
'first_name' => 'ASC',
)
);
}