phpmyadmin/sql-parser to AST

I have already asked a question here PHP Sql expression to AST, but my problem is not resolved. I have tried using the project https://github.com/phpmyadmin/sql-parser to parse my SQL expressions, but I am not able to get an AST of them. What am I doing wrong?

<?php

require_once(__DIR__ . '/vendor/autoload.php');

$expression = "   SUM  (   order_part.amount    *   0.8     )  AS `total` ";
echo $expression . "\n";

$parser = new \PhpMyAdmin\SqlParser\Parser($expression);
$parser->parse();

// ------------------------------------

$tokens = \array_values(
    \array_filter(
        $parser->list->tokens,
        function (\PhpMyAdmin\SqlParser\Token $token): bool {
            return $token->type !== \PhpMyAdmin\SqlParser\Token::TYPE_WHITESPACE;
        }
    )
);

$expression = \PhpMyAdmin\SqlParser\Components\Expression::parse(
    $parser,
    new \PhpMyAdmin\SqlParser\TokensList($tokens),
    [
        'breakOnAlias' => true,
    ]
);

echo \json_encode($expression) . "\n";

I am only getting this as an output:

{"database":null,"table":null,"column":null,"expr":"SUM(order_part.amount*0.8)","alias":null,"function":"SUM","subquery":null}

I see that "function":"SUM" and that is correct but I cannot go "deeper" into the expression, I dont see its parameters, sub-expressions, etc. To be clear, I would like to get an AST of the expression.