Sim's D is fine. One of the challenges is that we switch on everything. Zuby, Scott, RJ and Kadary are fine for that, they can all handle 1-5. But Sim is shorter and not as physically developed yet so he has difficulty in certain matchups.on the switch. Same with some screens as you noted.
The more concerning thing - and I say this as an enormous fan of Wilcher - is that the shot thing is clearly mental (on the 3s it's technical, he really does need to use the college line instead of the pro one) and that is coupled with a propensity for a few flat-out stupid plays every game.
I think both of those stem from trying toi hard, but by this point in his career I would have hoped he would settle in. Still time this year or next to become a very good player. But some warning signs there.