Some other good places are
Zilker Clubhouse. It's on the west side of Mopac & has two simlar viewing ares. One is off on the right-hand side of the road before you reach the clubhouse.
The other is from the patio of the clubhouse itself.
Zilker is best in afternoon/late afternoon. You'll be facing east and the sun will behind you.
You could also try Lamar Square Shopping Center. I think it's in the 1300 block of S. Lamar. There's a gym (Is it Steve's Gym? I can't remember) that has a second floor balcony (accessible to the public) that offers this view:
Lamar Square is also better in the afternoon/late afternoon because of the position of the sun.
If the Convention Center is open while you're here, go inside and up to the fourth floor. There's a big terrace and patios on the west and north sides of the buildiong:
The Convention Center is just west of I-35 and faces west, so morning is a good time to go there, although you can get some good night shots if the Convention Center is open after dark. (sometimes it is; sometimes it isn't)
If you're going to TMI Castle, you might as well hit the stairwell at this condo on the way.
It's on the north side of 12th St just west of Lamar.
You can go up to the top level of that stairwell on the left hand side and get this general view: