How to run a JS function after DOM is complete && server connect is done?

I need to execute a javascript function AFTER both:

• A server connect is finished loading
• The DOM is finished loading

How do I do that?

More info:

1. I have a function that initiates Tiny MCE. It replaces a textarea with the Tiny MCE editor.

function initTinymce(content) {
    // document.onreadystatechange = function () {
    //     if (document.readyState == "complete") {
    console.log('LOADING INIT TINY MCE');
    var globalContent = content;

    tinymce.init({
        selector: 'textarea#tinyEditor',
        setup: function (editor) {
            editor.on('init', function (e) {
                editor.setContent(globalContent);
                editor.save();
            });
            editor.on('change', function () {
                editor.save();
            });
        },
        min_height: 500,
        plugins: [
            'advlist', 'autolink', 'lists', 'link', 'image', 'charmap', 'preview',
            'anchor', 'searchreplace', 'visualblocks', 'code', 'fullscreen',
            'insertdatetime', 'media', 'table', 'help', 'wordcount', 'powerpaste'
        ],
        toolbar: 'undo redo | blocks | ' +
            'bold italic backcolor | alignleft aligncenter ' +
            'alignright alignjustify | bullist numlist outdent indent | ' +
            'removeformat | help',
        content_style: 'body { font-family:Helvetica,Arial,sans-serif; font-size:16px }'
    });
}

2. I have a server connect that loads data from the database, this data will populate the textarea that TinyMCE will initiate on.
   For example, it’ll populate: “My text from the database”.

3. I put a dynamic event ‘on success’ on the server connect, that does a RunJS to trigger the function:
   

   

   image459×722 19 KB

4. The server connect is finished faster than the DOM changes to the “complete” status (and thus load all the elements and the tinyMCE JS library).

5. The result: the editor isn’t loaded, it still looks like a regular textarea:

   

   image828×195 3.49 KB

If I run the function initTinymce() in console,it does load in the editor:

image873×561 12 KB

So I conclude that the issue is that the server action is loaded (which I need to pre-populate) but the DOM isn’t completed yet.

I need to run the function after BOTH the DOM is complete and the server connect is done. How?