As you mention, Overchannel is a 14th level ability. It is supposed to be crazy powerful. And you can only overchannel 1st through 5th level spells, which at level 14 can be significant damage but won't instakill most enemies of appropriate CR.
Potions of Speed and Quicken Metamagic are acquired at low levels in BG3. Two 3rd level spells (e.g., fireball) against enemies you face at 5th level is much more powerful than a maximized 5th level spell against enemies you face at 14th level.
Empowered Evocation is a 10th level ability, and doubling the damage of magic missile is...fine, but not extremely crazy. A 5th level magic missile will then do 7*(1d4+1+5)=60 points of damage, which is great but not amazing. For comparison, Cone of Cold is AoE that deals 8d8=36 average points of damage on a failed save. So Cone of Cold targeting 3 creatures will do more damage than an Empowered 5th level magic missile.
Quicken is much more powerful (relative to current character level and CR of enemies you'll face) than Overchannel or Empowered Evocation.
