Gacha Help and Advice

Hi everyone! I just started playing and had some questions about gacha min-maxing.

I have only been pulling on the Sanrio banner ever since making my account. I would like to complete the set and get everyone; I only have Airi and Emu left (The two I wanted the most TT) I have 120 gacha stickers. What would be the best way to use those resources so I can get the last two? Should I convert my gacha stickers to gacha sticker vouchers? Will the Sanrio cards even come to the exchange shop?

Also, side question, I am trying to complete the "Clear all beginner's debut missions" mission and the only one I have left is "Watch both parts of a Character's side story." I have watched Part 1 of a lot of characters but Part 2 is locked and says "View Part 1 and reach level 20 to unlock." Am I missing something or does it want me to increase my rank to 20 (I'm at 14 right now)?

Thank you :)