It's definitely doable for f2p to get Rares. I didn't buy a single seal scroll and just saved the ones from the events, GNW packs, Matsuri etc. Had enough for 6x10 pulls in a little over 3 weeks. Sure, it takes time but it's doable. It's super rares that are practically impossible considering they need around 400 pulls.
Oh and btw your Free pulls every 2 days still count towards those numbers.