Forgiveeee

Yes you can, i just did it 5 days ago. You need to own a 60-Ticket, log into the character you want to boost, press escape and one of the options in the bottom right corner lets you use your ticket. Everything else you said was right, IA and serpent quests are not finished yet and the last 6-8 story quests still need to be done. Not sure if it's allowed to post links but here's the official way: How To Use A "Level 60 Boost Voucher" On An Existing Character – Blade & Soul Support (bladeandsoul.com)