should I 5* +atk/-res m!morgan, Julius, or add merges to my nowi?

I would say upgrade Julius because of his skills that are very rare, as well as his unique and niche weapon that you could play around with and see what works best for him. If you're not dying for another red tome user, however, I would add merges to Nowi.

Merge projects come first in my opinion.

If you are looking for a really good niche unit, I would prioritize Arvis. He is very fun to play around with and quad ploys are invaluable, especially in grand conquests.

If you want offensive support, go Morgan. If you need defensive support, go Julius