Some people mentioned items that boost the caster dmg. What items are available? Haven't looked to much into the caster items and wanted to do some dpr analysis.
I personally like dual-wielding wands of manifold missiles. 1 action cast for magic missile, but then you get a free 1 action casting of magic missile every turn after that for 1 minute as long as you are holding the wand. If you have two, you can activate both and get 2 sets of missiles per turn. Or I suppose you could use the full 3 action casting variant on turn 1, and the same on turn 2, but the efficiency falls off that way.
At 11th level, its pretty easy to afford 2 of those wands at the 3rd spell level, so that 4 magic missiles per turn after the first (14 average extra damage per round). Alternatively mix with 2 casting magic on the 1st and 2nd round. The more rounds they're active, the more efficient they are.
Staff of divination gets you potentially a whole bunch of extra castings of true strike for spells that require attack rolls.