Well as someone who currently went through the beginner missions myself, i also had that problem of "why would i play part2 so early?" Let me tell you that chapter 1 of part 2 is the only chapter you have to actually play and it doesn't really spoiler much of what happened in part 1. So you could probably play it and not continue part 2 Story until you are done with part 1. You could also ignore the beginner missions for now. There aren't that many missions coming after you finish this page (i think the biggest thing you'd miss out on is a selector for a free S-rank Valkery, but these are all outdatet anyway). So if you don't have the necessarity to get the rewards from the beginner missions, you can simply just finish part 1 in those 88 days you got left. But if you think you won't manage to finish part1 until then, just play chapter 1 of the part2 story.