So I finished Ni no Kuni last night.
The good news is that the combat system gets a lot more manageable once your party size increases. The AI-controlled characters are actually fairly smart as long as you give them a good choice of familiars. I rarely had to mess with the Tactics panel (where you can give them basic instructions on how to act in combat) or directly control another character.
I tried to do every errand and bounty hunt before moving on with the respective story quest, which occasionally had me fighting trash mobs for hours (sometimes in vain, as it turned out I already had the rare drop I was looking for), but also ensured that I was well prepared for the next dungeons. In this regard, Ni no Kuni is a very typical JRPG, where grinding makes for a smoother ride.
However, I found that the side quests lacked creativity. Healing brokenhearted people in particular got very old very quickly, especially when you had to keep treating the same people for different "heart conditions" in different acts.