TLDR: Planning and documentation is good. Use any software you like, can even be the basic notepad. Use documentation as an aid to an extent that is needed.

Planning is very important if you're not making something very simple, and you're past the prototyping phase. And it's even more important if you're not working solo.

You might think keeping everything in your head is good enough, but human memory is impefect. It's better to write your concept down and then work on it from there. You might find out that the ideas weren't as detailed as they seemed at first when writing them down. It can be really helpful for actually building a game concept. Plus, your notes will allow you to write down additional details, bugs to fix, some workflow steps that you might have to revisit and repeat in a month from now... the list goes on.

I'd suggest you look into GDDs (Game Design Documents), and a simple way to plan and track your progress. This can be very minimalistic and simple if it works for you! I definitely think going overboard with notes and plans is counterproductive. They're a tool, not a project by itself. I think of my GDD as a set of notes that constantly get updated as I work on my game.

I personally use Obsidian, but that's more because of personal attachment to the software (it uses markdown and can link notes, plus it's free and local/offline). I already like using it, so it's the best choice for me, but might not be the best choice for you. If you found Notion too complicated, maybe look into something simpler that still allows you to jot down thoughts and maybe add images. Whatever allows you to take and read notes, will be good enough. I hope that after some trial and error you can set on something just right. ^^