chain.php

<?php declare(strict_types=1);

$app->get('/prices', [
    ParseParams::class,
    NormalizeParams::class,
    delayed() => Notify::class,
    parallel() => [
        ServiceA::class,
        SomeOtherService::class,
        ServiceB::class,
        serial() => [
            ServicePrepareC::class,
            ServiceC::class,
        ]
    ],
    MergeResults::class,
    Sync::class,
    Response::class,
]);

index.php

<?php declare(strict_types=1);

// ¯\_(ツ)_/¯