With Simmons, I'm pretty confident he'll never be a good pull up shooter, he's just too unnatural and far behind. However, I'm not one of the people that will crucify him for his lack of jumper. The last thing I want him to do is chuck up mid range shots at a crap percentage, mid range shots are good if you could hit them in the high 40s at least, and not many players could do that.
Right now he's a very efficient 58.1% true shooting percentage. If he makes a reasonable and very possible improvement at the free throw line in the upcoming years, to say 70%, that would be great. For reference if he shot 70% from the line this year his TS% right now would be 60.0%, which would rank him 18th instead of 28th amongst the 64 players averaging more than 16 PPG this season.
Clean up the turnovers a bit and get him in a properly constructed lineup (two versatile guards+Harris+BS+JE) as well. His FG% at the rim is some low hanging fruit to improve upon too. In the distant future, maybe he could work in a set shot 3 pointer, but right now it should be about those free throws. Basketball, and pretty much all sports, are all about the value a player adds, if Simmons is adding great offensive value I don't care if it's not the conventional way a typical primary ball handler does it.