Balance when you all know the difficulty/challenge difference from quest to quest is not linear. Think waterworks, the last quest being the easiest or the various catacomb quests.
Add to that character power progression 'jumps', Monsters too get new spellls and weapons that do multiple damage types..
Even though you have no spoilers and some hav'nt 'seen it all'. This is dungeons and dragons, you hear 'tavern talk'. It's not unreasonable that your 'characters' have heard 'tales' of easy/difficult quests. All 'normal' dungeons.
I can understand balancing trap skills as these are pretty linear except a few 'master' locks and traps. Personally its about seeing and discovering the trap much more than disabling it.
Yes spellcasters on normal are powerful or prone to overkill untill their sp run's out. The new SLA's are good but they are on timer's and don't do as much damage. Don't forget the variety of spells and what effects they do. To some extent monster save's will be linear like your character gets higher saving throws, but then you have the Monster Manual with many tricks to spoil your day . Also remember that dastardly Grondley could'nt do the quest without his friends :-). So balancing spellcasting with what, easy/long(quests), undead, demons, fire, charm get my drift. You'd have more fun trying the long/difficult and often more interesting quests. Was the Red Fens fun did you do them all ? Surely each quest had its own challenge.
If your arcane's are killing things easily, try using spells such as blur/aid/stoneskin/haste/web as you would in pnp, what you would'nt do in pnp is cast a fireball into the middle of combat!
P.S. Waiting for a story of what happened in last nights adventure.