help with AJAX in plugin (500 bad request error)

I am getting into plugin development and am currently familiarizing myself with sending data via AJAX.

The goal is to sent a variable that is in my javascript file to my php so that i can store it in the database (currently I only want to echo it out before taking any next step). I created a very simple plugin with help of the wordpress codex and some online tutorials.

However, i keep being unable to get it to work. Every time I sent the AJAX request I get a 500 internal server error and a bad request error.

Can someone help me out with what mistake I’m making in my request and request handling?

// php code

<?php
/*
Plugin Name: ajaxtest

*/

// Enqueue script.js
function simple_ajax_enqueue_script()
{
    wp_enqueue_script('ajax-script', plugin_dir_url(__FILE__) . 'script.js', array('jquery'), '1.0', true);

    $my_ajax_nonce = wp_create_nonce('ajax_example');
    wp_localize_script(
        'ajax-script',
        'my_ajax_obj',
        array(
            'ajax_url' => admin_url('admin-ajax.php'),
            'nonce'    => $my_ajax_nonce,
        )
    );
}
add_action('wp_enqueue_scripts', 'simple_ajax_enqueue_script');

// Shortcode to display "Hello, world!"
function ajax_test_shortcode()
{
    return '<button id="test-ajax-button">Test Ajax</button>';
}
add_shortcode('test', 'ajax_test_shortcode');

/**
 * AJAX handler using JSON
 */
add_action('wp_ajax_my_tag_count', 'my_ajax_handler');
add_action("wp_ajax_nopriv_my_tag_count", "my_ajax_handler");

function my_ajax_handler__json()
{
    check_ajax_referer('ajax_example');
    $datastring = wp_unslash($_POST['sendingdata']);

    echo $datastring;
    wp_send_json("data was sent successfully");
}

jQuery(document).ready(function ($) {
    $('#test-ajax-button').on('click', function (event) {
        event.preventDefault(); 
        var datastring = "hello world";
        $.post(my_ajax_obj.ajax_url, {
            _ajax_nonce: my_ajax_obj.nonce,
            action: "my_tag_count",
            sendingdata: datastring
        }, function (response) {
            console.log("Response:", response);
        });
    });
});