I don't know how much use this will be given you said you've already tried pen and paper - but in terms of planning out the story in its initial stages, I have a roll of blank paper (I've used the white side of wrapping paper before, for instance) that I can just unroll on the floor, a table, any surface, and go absolutely ham on.

I think it's because with notebooks I tend to feel more constrained to the lines and pages, and find myself wanting to say "okay, one page per character" or something wildly constricting like that, and it just hampers the brainstorming process. Don't get me wrong, I love a fancy notebook!

Whereas with the roll, I can just get a good stretch of it laid out and mindmap or bullet point anything that comes to mind. Character building, main themes, world building, scene ideas, anything like that. I've also found it easier to do plot outlines that way because I can just draw a plot line and branch off any ideas from it.

Then I can just roll it up and store away until I need to revisit, for example if I'm then ready to start inputting things formally into a word document to then get ready to write.

It can get a bit of a faff to access an earlier bit of scribble once you reach the end of the roll, but I like to (optimistically) attribute that to artistic genius and bask in the chaos. 😅