Mitchell Robinson just needs minutes to do damage, end of story. He proved that last year and he's not going to get worse his rookie season after an offseason of training with NBA coaches and staff. Only obstacle in his way is Fizdale, who in my opinion is a bottom 3 coach in the league.
I also find it strange that you feel a weakness is that he feels uncomfortable with the ball 5 feet out of the paint. That is not his strength, nor is that Gobert/Drummond/Capela/Deandre/Adams/Whiteside/McGee's strengths. Not everyone is a Jokic, Gasol, or Vucevic and that's fine.
Robinson was a top 25 player the last few months in the league when he came back and was a league winner if you got him off the wire. I personally would avoid him simply because of the logjam and the fact he's on the Knicks but beyond that, he's going to be a top fantasy player for years to come simply because of his proven ability to get you 4+ blocks on any given night. Sure he can turn into a Larry Sanders type of guy that fizzles out but we'll see.