I have managed to upgrade all my WA sites now to php8.2 which included one eCart site with very few issues - any problems I had have already been covered in a separate thread at https://www.webassist.com/forums/posts.php?id=46999.
If you can copy the site onto a testing domain running php8 then add error reporting at the top of any page that throws an error, I found that's the best way of dealing with it:
<?php
error_reporting(E_ALL);
ini_set('display_errors','on');
?>