Year 1 - defend and rebound
Year 2 - roll and score on lobs
Year 3 - jumper and hand offs
While his jumper obviously needs work, is Williams, as of right now, really incapable of doing everything else that you've listed?
He will show flashes in all of these areas, including his jumpshot, but they will all be inconsistent.
The nuances of defensive positioning might take time. One play he might have an amazing switch on to a guard, force them into a fall away long 2, and then block it, but then the next play he might lose his man for an easy two.
The nuances and feel of the pick-and-roll (something Horford is the best in the game at) will be difficult too, because he just didn't have the reps as Texas AM. He'll need to learn the speed at which to set a screen, when to re-screen the other side, when to fade, when to dive hard, and how to do all of that without getting offensive fouls.
Luckily, every time he sets a screen for someone, that person will be a high-level offensive threat, from Kyrie to Hayward to Rozier to Tatum. That means the defense will focus more on the ball-handler than him. He might look really good because he won't command as much attention as others.