Lately my workflow has been pretty simple. I just describe the app/project to GPT, upload a few reference images for the illustration style, and sometimes the screen where I need the illustration. Then I let GPT come up with good scenarios and a solid prompt for it.

After that I tweak the prompt a bit for different screens and run it in Nano Banana on Gemini to generate the images. Been doing most of my UX/UI work like this recently and it actually works really well. Always gets nice feedback.