How to reference argument data for wp_nav_menu() from functions.php

a function like this:

function get_nav_args($overrides = array()) {
    $defaults = array(
        'theme_location' => 'main-menu',
        'menu_class' => 'list-inline',
        'add_li_class'  => 'list-inline-item',
    );
    return shortcode_atts($defaults, apply_filters('some_custom_identifier_nav_menu_args', $overrides, $defaults) );
}

would be callable like wp_nav_menu(get_nav_args())

using shortcode atts and allowing an overrides array to be passed in would allow you to change the values if necessary without duplicating the whole thing. As for your put everything in functions.php I would avoid that if possible. Use include or require to include files that contain your functions. this way you can sort them out by type, use, location, or whatever else you’d like and won’t end up digging through a single massive file looking for something down the road.