I can see a lot of the flaws you're pointing out. Yet, I don't think there's room to argue with the notion that the Celts exceeded expectations every season under Brad until this year. So I think the key question here is what changed? Is it simply the burden of expectations? Is it that Brad is coaching a team with an excess of talent for the first time?
I will say that in watching the game against the Cavs last night, I felt that the Cavs were actually running a better offense than the Celts for much of the game. The Cavs simply didn't have guys who could hit shots. But in the 1st quarter especially it felt like the Celts had a lead only because they were hitting tough shots and the Cavs missed a number of quality open shots that their ball movement generated. I liked the way the Cavs swung the ball around the perimeter and had good floor balance with a big man diving to the basket while other guys fanned out on the wings.
The Celts have to lead the league in drives that go nowhere. The Celts don't exactly drive the ball very often, but I feel that when they do, it seems that a lot of the time the guy driving doesn't have an apparent plan, just barrels toward the paint, apparently hoping that something will open up when he gets there. Part of that, I feel, is that the Celts tend to play 5-out sets instead of playing 4-out sets with a big man rolling or in the dunker spot. That can tend to leave the guy driving into the paint without an obvious, large target to pass to if the defense collapses on him.
I don't mind 5 out sets, to be clear, but I think they work best if you have a really great driver (e.g. Giannis) and you surround that guy with very good shooters. The Celts don't really have a great driver on the roster unless / until Jaylen, Jayson, or Hayward becomes more consistent, and most of the shooters that they put out there are just okay.
I agree that the hallmark of a Popovich team is allowing players to play to their strengths. Before this year, I thought Stevens followed the same philosophy.
Perhaps the issue is that he does. Could it be that the team that Ainge has constructed for Brad is comprised primarily of players who want to get their offense out of isolation sets? Kyrie, Jaylen, Tatum, Rozier, Morris, and even Hayward fall into this category. Even Al Horford likes to break down mismatches with a one on one fairly regularly, though I think he mostly does that when he has a major mismatch.
Maybe Brad has been trying to let this team play to its strengths, the problem is that the natural tendency of more than half of the rotation is to go hunt for shots 1-on-1.