If I could give you only one piece of advice it would be this: Get started on this process sooner rather than later. Some methods take more time than others and deciding what you want to keep will take longer than you think! Or maybe exactly how long you think and that's why you're avoiding it, but it isn't going to get any easier (or shorter!)
You can identify the files/folders you want to copy by starring ⭐ them and/or changing the color.
Star files/folders in Google Drive by right clicking on the item and selecting "Add to Starred" OR by single clicking on the item and pressing the "s" key on your keyboard.
Color code folders by right clicking and selecting "Change color" from the menu.
NOTE: When you download Google Docs, Sheets, Slides, etc. they are converted to Microsoft file formats (Word, Excel, PowerPoint). You can change your Drive settings to automatically convert uploaded files back to Google Docs editor format if you choose.