It's much easier to make use of your brainstorming later if you write it all down. Like, literally write the entire thought process. All your justifications, waffling, everything. Connect brain directly to fingers.

You need a method to quick jot stuff down in the moment. I use google keep for those thoughts/ideas. It's on my phone, very low friction always to just pull it out and get something out of my head. Make it good later, just use your notes app (or an actual notepad if you're oldschool) to get it out of your head so you can think of other stuff without worrying about losing it.

Lore =/= story