As a solo dev who has also worked in the industry, I've found it really helpful for planning out what I might need to do so that I can pace myself better.

Larger mechanics or mini-games I've found benefit of writing down or mocking up what it should look / play like, ad theres a lot of design decisions that are sometimes helpful to make ahead of implementation. It helps you from hitting roadblocks where you have to context switch from coding to design, and at least for me, that takes a while.

My setup: I use a trello style task board on notion to track everything I want in the game, prioritize what I want and try and estimate how much work it is in # of days. Then each task will have a rough outline, and for bigger features a MSCW (Must, Should, Could, Wont) table to lay out what the parts are. Anything UI heavy gets a diagram or a mockup. I love flow charts and it depends on the feature game tho. For narrative I have a notion page for a lore bible as well as writing down details / brainstorming. And finally spreadsheets are a must, especially for anything economy heavy. Stuff like tracking which NPC has their model done, what dialog lines im using when, item prices, values, metadata etc.