Perhaps use a nav_walker or a filter on wp_nav_menu_objects
Here is a simple nav_walker which, when output on wp_nav_menu will wrap the contents of each link in the menu with a span
<?php
/**
* Add_Span_Nav_Walker
* wp_nav_menu walker-class to wrap contents of each menu-item link in span element
*/
class Add_Span_Nav_Walker extends Walker_Nav_Menu {
function start_lvl( &$output, $depth = 0, $args = [] ) {
$indent = str_repeat( "\t", $depth );
$output .= "\n$indent\n";
}
function end_lvl( &$output, $depth = 0, $args = [] ) {
$indent = str_repeat( "\t", $depth );
$output .= "$indent\n";
}
function start_el( &$output, $item, $depth = 0, $args = [], $id = 0 ) {
$indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';
$class_names = $value = '';
$classes = empty( $item->classes ) ? [] : (array) $item->classes;
$classes[] = 'menu-item-' . $item->ID;
$class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );
$class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';
$id = apply_filters( 'nav_menu_item_id', 'menu-item-' . $item->ID, $item, $args );
$id = $id ? ' id="' . esc_attr( $id ) . '"' : '';
$output .= $indent . '';
$attributes = ! empty( $item->attr_title ) ? ' title="' . esc_attr( $item->attr_title ) . '"' : '';
$attributes .= ! empty( $item->target ) ? ' target="' . esc_attr( $item->target ) . '"' : '';
$attributes .= ! empty( $item->xfn ) ? ' rel="' . esc_attr( $item->xfn ) . '"' : '';
$attributes .= ! empty( $item->url ) ? ' href="' . esc_attr( $item->url ) . '"' : '';
$item_output = $args->before;
$item_output .= '<li' . $class_names . '><a' . $attributes . '><span>';
$item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
$item_output .= '</span></a>';
$item_output .= $args->after;
$output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
}
function end_el( &$output, $item, $depth = 0, $args = [] ) {
$output .= "\n";
}
}
When we output our nav_menu in the them we can use Add_Span_Nav_Walker
as the walker class:
<?php
if ( has_nav_menu( 'wpse288486_nav' ) ) :
wp_nav_menu( [
'theme_location' => 'wpse288486_nav',
'walker' => new Add_Span_Nav_Walker(),
] );
endif;