Next mystic codes to level up

I'd recommend leveling Atlas Institute next

Atlas Institute is nice but the levelling on all 3 skills doesn't really scale well, and you'll only be working on the the -2 cooldown to level 10. It's already very powerful on level 1 and is something you use for challenge runs or specific fights, which the base level already does a lot for.

For something you can level up quickly and if you got a decent amount of Quick-based servants, go with Royal Brand. Highly recommend getting Mage's Association leveled up overall next, the group heal is appreciated and while the 20% NP isn't too much, it can help you get out of a pinch. The card reshuffled too can get you out of a bad hand.