Sylvan Library comes out against ANT / Storm in my experience. Against fast combo decks you need something with an immediate effect on the game when it is played. You don't have time to accumulate advantage with Library in those matches.
@ Lost Legacy
Rather than Lost legacy, I really like
sadistic sacrament. It is slightly harder to cast but can also be better when you want to cast something but are not sure what to name. Against storm/ant it can be better since they have few threats but with various mames. Many times 3 cards is enough to get pesky blood moons or other problematic sb cards. Combined with discard it will often eliminate the 4 copies you would worry about. I personally dont think its necessary for the deck but id play it before lost legacy.