You should use your custom plugin.

Copy from /mod/custom_index/views/default/custom_index/content.php

to /mod/my_plugin/views/default/custom_index/content.php

And add the blocks:

// discussions
if (elgg_is_active_plugin('discussions')) {
       $discussions_params = $list_params;
       $discussions_params['subtype'] = 'discussion';
       echo elgg_view_module('featured',  elgg_echo('collection:object:discussion'), elgg_list_entities($discussions_params));
}


// pages
if (elgg_is_active_plugin('pages')) {
     $pages_params = $list_params;
     $pages_params['subtype'] = 'page';
     echo elgg_view_module('featured',  elgg_echo('collection:object:page'), elgg_list_entities($pages_params));
}

The Argon Theme is only compatible with elgg 2.3.x:

https://elgg.org/plugins/3110405

I don't have an estimated date for a new release compatible with elgg 4.x

Thanks

Thanks