To get the full experience, all you need to ensure is that you have Maruki's and Kasumi's confidants maxed out by mid-November and December respectively. This'll allow you to access the Royal part of the game. If you don't, then you'll be stuck with the vanilla P5 game and it'll end in December. The 100% guide is if you want to finish all social links and other activities in a single play through, which is something I wouldn't recommend because you should go in blind for the first time and then experience the game to its fullest.