It's not just getting 6th level magus spells early though, it's getting 9th level lich spells early. And you know, sure it is very strong for a wizard. But at the same time, a wizard benefits quite a bit less from the various mythic feats*. And a wizard is pretty squishy. A magus is not very squishy, gets free metamagic, and can dish out insane damage.
Again, it is a single player game, so you can do what you want. If you want your character to start with 20 in all stats, or increase the level cap to 30, that's fine. But don't pretend that just because there's a very strong thing that can be done under certain conditions, that you are now making available under much less stringent conditions, that this isn't making the game easier, and potentially destroying the challenge.
*There's obviously good stuff for casters, but at the same time most of it is like: yeah, cast more spells per day, or have your spells last longer. Convenience, not power. Or mythic feats that are just mandatory like ignoring energy immunity or spell pen. The mythic metamagic stuff is very strong though.