My gut is that Sexton’s defense does not really get anywhere close to league average. He’s not a dumb defender. I.E., he knows where to be, but his size is a problem, especially when you play him with Garland.
I do believe that Sexton is going to improve his scoring efficiency, continuing the trend from this season. As a scorer, Sexton is going to be a problem because he will be a very efficient player that will torch teams on switches.
The area I see that is the biggest question mark is Sexton’s intuition of offense. He is currently pretty mediocre at this (which, ironically, goes to show just how elite his 1v1 scoring is). If Sexton can learn to know where the opposing players are on the court, he will be able to compensate for poor vision. Also, if he learns to play off-ball, that will create switching nightmares for opponents.
So, to me, his ceiling is an elite scorer who uses off-ball movement to make his teammates better, that can be a secondary facilitator, while also being a slightly below average defender for a point guard. Maybe a Chauncey Billups between 2003-08 level player?
I think the most likely outcome is somewhere between an Eric Gordon and Jason Terry. Basically, a guy that comes off the bench, but regardless of his limitations, always will Play significant minutes because he is unguardable.