It is already written, and that settles nothing

Yes, the result is known to Allah. That is true of everything, including what you will eat tonight, and nobody uses it as an argument against cooking.

A statement about who decides the outcome is not an instruction about what you should do. If it were, the same logic would make eating pointless, and seeing a doctor pointless, and locking your front door pointless.

The decree includes the means. Your revision is not outside the thing that is written. It is part of it.

Ibn al-Qayyim closes that loop from the other side in the same book: فالتوكل من أعظم الأسباب التي يحصل بها المطلوب ويندفع بها المكروه فمن أنكر الأسباب لم يستقم منه التوكل. Our rendering, rather than a stored translation: tawakkul is among the greatest of the means by which what is sought is obtained and what is disliked is repelled, so whoever denies the means, tawakkul is not sound from him. Reliance is not the alternative to the means. It is one of them.