Hm. Good points all around.
Perhaps moving hitdie up to 1d8 and BAB to full would make it a bit more appealing, along with cutting the requirements down to 3rd level arcane spellcasting and only 5 ranks in discipline and intimidate. I know the strength, constitution, and charisma requirements are a bit unusual, but they do seem to fit the theme quite well and suit the class to a sorcerer/monk or bard/fighter combo. It's absolutely more flavor than raw power.
If that's not enough, perhaps instead of crippling spell slots that badly, simply enforce the first spell slot each day to be filled with Mighty Flex, then leave the rest normal. Considering that's still giving wizard pace spell growth and +4 total to spellcasting stat, I'm pretty sure that's not a horrible restraint.
As mentioned, no armor and gloves/gauntlets largely for the purpose of being able to punch oozes and the like. Although it wouldn't be much of a stretch at this point to say the Muscle Wizard can punch whatever he wants with no ill effects because muscles.