I'm always sad when I get near the end of a good game or book 
You can definitely do the last two missions at level 19. Iirc it's recommended to be at least level 17. Cory scales up some though, so even doing it at a higher level should have some challenge to it. One thing I'd recommend is make sure you have any of your inner circle quests you want to do done before you start "Doom Upon All The World" because they won't be available after.
I know right? When an amazing series of books or games is finished, I get really sad because you sort of get into a certain mindset and get immersed in this new world and to have it end even though you want to know more... it stinks! Same goes for movies!
I now leveled up to 20. I just had to ask because I really do not have man missions left to do so I rather just get these final two over with instead of wandering around Ferelden or Orlais. Just gotta rip it off like a band-aid I guess!