Unable to use Factory::getDbo() in an AJAX call inside a custom Joomla 4 component, but it works fine in a custom module

Question

I am running a Joomla 4 website. I have a jBridge.php file that has the following:

use Joomla\CMS\Factory;
$session    =& Factory::getSession();
$document   =& Factory::getDocument();
$user       =& Factory::getUser();
$jdb = Factory::getDbo();

Inside my custom module I can use:

include("libraries/JBridge.php");
$jdb->setQuery("SELECT * FROM example_table");

And this all works fine. But when I try the same technique in my custom component as part of an AJAX call, I get an internal server error shown on the network tab of my browser.

From Googling I've found that I need to include a combination of the following:

include ("../../../../includes/defines.php");
include ("../../../../includes/framework.php");

This stops the internal server error on my AJAX call, but then no data is being returned in the response section on the network tab of my browser. Even if I just echo out "test".

I'd really appreciate it if anyone could offer any advice or point me in the right direction please?

Thanks.