I noticed that the spells at higher level really tend to break the game.
May I suggest .. cutting all lvl 6-9 spells from the game. Leave the spell slots, so they can be used for lower level spells (with meta magic enhancements). This also simplifies the late game (for dm & player alike).
But as compensation for players…
1) Gain a free metamagic feat when you reach a level when you first learn a 6-9 spell (4 bonus feats total)
2) Allow players to “learn” spells with metamagic enhancements already applied. So you can cast them without the extra casting time normally associated with metamagic feats.