You may be right, I'll admit I don't pay that much attention to shooting mechanics. My memory of rookie-year Keegan though is that he shot a lot of movement threes where we was inevitably turning and fading away to one side or the other has he rose up to shoot and he's had that one foot in front of the other stance and slightly flat shooting arc since college.
Maybe it's because I've seen so many guys with perfect shooting mechanics fail to capitalize on them at the NBA level (Ben McLemore and Jimmer Fredette spring to mind) but I'm inclined to say that a player who has already shot 41% on 500 three point attempts over the course of a season should keep doing whatever they're already doing. By trying to "fix" things you may just force that player to overthink their mechanics and actually make their shooting worse.
I think a few things for Keegan might be:
1. Added defensive responsibility. Playing PoA defender the way he did last year, often vs much smaller guards, requires a different level of conditioning and legs. We pretty much hid him as much as possible on D his rookie year.
2. Added offensive responsibility. His rookie season offense was very simple. Catch and shoot and movement 3s off the DHO. Now, we're asking him to create off his own bounce far more and I think you see some of that processing when he gets the ball. Instead of just letting that thing fly, you almost see that split second hesitation of "should I attack? Or..." and by then it's too late.
That last possession vs Utah was a great example. Fox hit him wide open for a step on 3, he hesitated thinking about a dribble move, and lost the space. First and foremost for him, especially with DDR aboard, is he's got to be ready to let that thing fly. I want him top 5 in 3PA this year is the NBA