There are mods on nexus, although you would have to restart. They mess up your game in progress pretty bad. And yes, giving us level 5 for EA only makes sense. Larian can't seriously be planning to go live without us testing all those high level wizard spells.
High level spell is wish, stop time and etc.
Fireball is low level. And yes, this game despite being a game where you escape a spelljammer mindflayer ship and enter on underdark on ch 1(on bg2, underdark is ch 5), on gameplay this game is a "dungeons & kobolds", not "dungeons & dragons" game.
Lv 5e on 5e is when a Wizard START to get some interesting spells. Necromancers gain animate dead, Evokers gain fireball... On BG2, a necromancer only get animate dead at lv 9, the same lv which paladins start to get their spells on 2e.
