Query and display only by first letter of the get_field value

Hope the below code block will help you. Please read the comments carefully. The code block-

// Your previous code.
// Say this is your $oder_terms variable
$order_terms = array(
    'Sanchez'   => 'Sanchez Link',
    'Smith'     => 'Smith Link',
    'Dramatist' => 'Dramatist Link',
    'Rashed'    => 'Rashed Link',
    'Munez'     => 'Munez Link',
    'James'     => 'James Link',
    'Jacky'     => 'Jacky Link',

);

ksort($order_terms);
// After ksort($order_terms); we get below array
/*
Array
(
    [Dramatist] => Dramatist Link
    [Jacky] => Jacky Link
    [James] => James Link
    [Munez] => Munez Link
    [Rashed] => Rashed Link
    [Sanchez] => Sanchez Link
    [Smith] => Smith Link
)
*/
// Now we need to group them on the basis of alphabet. Right ?

$new_order_terms = array();
foreach($order_terms as $key => $value) {
    $firstLetter = substr($value, 0, 1);
    $new_order_terms[$firstLetter][$key] = $value;
}

// Now if you do print_r($new_order_terms); your output will be
/*
Array
(
    [D] => Array
        (
            [Dramatist] => Dramatist Link
        )

    [J] => Array
        (
            [Jacky] => Jacky Link
            [James] => James Link
        )

    [M] => Array
        (
            [Munez] => Munez Link
        )

    [R] => Array
        (
            [Rashed] => Rashed Link
        )

    [S] => Array
        (
            [Sanchez] => Sanchez Link
            [Smith] => Smith Link
        )

)
*/
// All grouped by their first letter.