I tend to struggle with making dua to Allah swt after my prayers. In general I struggle speaking to him verbally. I prefer to speak to him silently in my mind. Would it be okay for me to write in my journal to Allah swt? Whether it be complaining or making dua or saying thank you or anything for that matter. Can I write to Allah swt?

Yes you absolutely can write to Allah. Feel free to do that if it is effective for you. It’s even Islamically recommended to do so.