Currently when enabling the Delay JS option it causes many many problems and errors. In some cases sites dont load properly, menus stop working, even scripts get broken. In some way, it acts like the Cloudflare Rocket option : It causes problems.
On the other hand, having an option to manually declare which scripts should ONLY be delayed, would make it easier to manipulate this otherwise fantastic option. It would be like having the plugin flying scripts within WP Rocket.
I believe WP Rocket can implement such an option and give more power to the user. It will make sure that the page speed is as optimized as possible.