There isn't any extra rewards for the completing the whole Reward Book Page.
(But I think, this event is generous enough, we don't need any more reward. I never thought, that I can get so many rewards/trinkets/cosmetics for MP...and the MP reward for logging in is insane! 400 MP for login, 200 for quests and 30 for wins! Every two days you get a Premium Legendary as a gift...only for log in. Unbelievable! Thanks CDPR)
Thank you for asking the initial question. This is good information to know in case you wasn't too invest the money into premiums instead of festive non animated borders and titles.