How to load PHTML file last after page is fully loaded

If it's a script, you can try wrapping your script code inside

$( document ).ready(function() { // Your code }); 

This will ensure that your code written inside will be reflected once the page is loaded.

domReady! - You can use this feature of the RequireJS like the native Magento's modules use:

<script> //<![CDATA[ require( [ 'jquery', 'domReady!' ], function($) { // Your Code }); //]]> </script> 

DOMContentLoaded - Executes after DOM is loaded (before img and css):

document.addEventListener("DOMContentLoaded", function(){ document.getElementById("demo").innerHTML = "My First JavaScript"; }); 

load - Executes after everything is loaded and parsed:

window.addEventListener("load", function(){ document.getElementById("demo").innerHTML = "My First JavaScript"; });