I agree. I have been a supporter of his since the draft, as well, and still think we got him at a steal (in terms of draft position). What I saw from him at the NHL level was a lot of second guessing himself. He would get a good shot opportunity and vice grip his stick restricting his natural shot (usually resulting in the puck being shot straight at the goalie if on net). Somewhere along the line a lot of self-doubt crept in which kept him from being the player we know he is capable of being. I think some of that falls on the coaching (from what I have seen he got a lot of negative reinforcement early on which was meant to motivate him to improve his two-way game and other aspects but backfired) and, like you said, goalies being at another level that he was not used to when coming to the NHL. I also think him sliding to the 2nd round in the draft was the start of it. It almost seemed like in that moment, as he sat there waiting for his name to be called deeper and deeper in the 1st round while his linemates went early, he started to build a mentality that others were against him (maybe that mentality was always there).
At the end of the day, he needs to believe more in himself and rebuild his confidence while both continuing to work on being adaptable and trust in his new coaches. It's ok to have a chip on ones shoulder, but it really seems like that chip has been hampering him from growing.