I always heavily document before coding. My preferred tool is just Obsidian. I just make a folder and take large numbers of markdown notes for everything.

Heirarchal notes let you organize however you want, you can make your own little wiki to handle the zooming.

I also am very much considering getting some kind of digital drawing device, like a Boogie Board with the smart pen, or else waiting till there's an affordable stylus phone that I like(The Moto Stylus is the only recent cheap one and that lacks wireless charging).

But for now I primarily rely on text for everything, until I'm ready to move into CAD or code or whatever I'm doing.