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I managed to get this working using nothing but WordPress filters, thanks to @Milo's hint.

I created 4 filters:

• 1 to modify JOIN (to add wp_postmeta)
• 1 to modify WHERE for the Previous link
• 1 to modify WHERE for the Next link
• 1 to modify SORT

Here's what I came up with, it seems to be working but if anyone spots any issues I would love feedback:

function get_adjacent_past_events_join($join) {
  if(is_singular('event')) {
    global $wpdb;
    $new_join = $join."INNER JOIN $wpdb->postmeta AS m ON p.ID = m.post_id ";
    return $new_join;
  }
  return $join;
}
add_filter('get_previous_post_join', 'get_adjacent_past_events_join');
add_filter('get_next_post_join', 'get_adjacent_past_events_join');

function get_prev_past_events_where($where) {
  if(is_singular('event')) {
    global $wpdb, $post;
    $id = $post->ID;
    $current_event_date = get_field('date', $id);
    $today = date('Ymd');
    $new_where = "WHERE (m.meta_key = 'date' AND (m.meta_key = 'date' AND CAST(m.meta_value AS CHAR) < '$current_event_date')) AND p.post_type = 'event' AND p.post_status = 'publish' AND (m.meta_key = 'date' AND (m.meta_key = 'date' AND CAST(m.meta_value AS CHAR) < '$today'))";
    return $new_where;
  }
  return $where;
}
add_filter('get_previous_post_where', 'get_prev_past_events_where');

function get_next_past_events_where($where) {
  if(is_singular('event')) {
    global $wpdb, $post;
    $id = $post->ID;
    $current_event_date = get_field('date', $id);
    $today = date('Ymd');
    $new_where = "WHERE (m.meta_key = 'date' AND (m.meta_key = 'date' AND CAST(m.meta_value AS CHAR) > '$current_event_date')) AND p.post_type = 'event' AND p.post_status = 'publish' AND (m.meta_key = 'date' AND (m.meta_key = 'date' AND CAST(m.meta_value AS CHAR) < '$today'))";
    return $new_where;
  }
  return $where;
}
add_filter('get_next_post_where', 'get_next_past_events_where');

function get_adjacent_past_events_sort($sort) {
  if(is_singular('event')) {
    global $wpdb;
    $new_sort = " GROUP BY p.ID ORDER BY m.meta_value+0 DESC";
    return $new_sort;
  }
  return $sort;
}
add_filter('get_previous_post_sort', 'get_adjacent_past_events_sort');
add_filter('get_next_post_sort', 'get_adjacent_past_events_sort');