A few thoughts...
Your control room needs to be symmetrical. You've got the right idea angling the left wall to avoid standing waves, but you need to build a false wall on the right to match it. Otherwise you'll screw up your stereo image. You'll also probably need a bass trap on the back wall for that size room.
You can get some great sounds from a nice big room. Why don't you treat the walls and ceiling of the garage with some 703 and buy a rug that you can roll up when you're not tracking? That way you could have 2 iso booths and a nice little live room for drums.