When you cast a leveled spell using a bonus action, the only other spells you can cast with your action on that turn is a cantrip.
I, personally, think it’s confusing and doesn’t really add much in the way of balance to the game. Let the wizard burn all his spells twice as fast and be useless for the rest of the adventuring day. If your adventures have meaningful consequences for taking too long clearing a dungeon, it’ll work itself out.