This is the big thing to me. Mccullar's jumper at Kansas already improved by a leap because of better spacing as opposed to his years in Texas Tech. NBA spacing is only gonna be better. So guys with good shooting mechanics but longer wind-ups can sometimes take a step forward just by being in a more open space so they can fire off (though, admittedly against better athletes, that space sometimes doesn't help).
Plus, as you said, the one image we got from the G-League was an off-the-dribble shot with a shortened up release. I'd expect him to continue to simplify aka shorten that release into something he can more easily repeat. So factor in that he already seemed to round a corner with the Jayhawks, and should get even more work in at Westchester, and I'm very optimistic that he can become a consistent shooter. Not an elite or great shooter, but a reliable one.
Tbh, not great IMO. The best note I had from the handful of games I watched was that he stayed aggressive. But his athleticism doesn't get to show because his handle is too limiting to build momentum to the rim. His jumper never looked great in the games I watched. His passing was simple and safe, which makes sense with a shoddy handle. I give him props for defense, effort, and aggression...he's clearly trying to do the things that he'll need to do at the next level. But the last time I watched him, it felt like he was a year away from being a year away.