I agree with your assessment of the current roster. I don't quite agree with you that Amir is a "decent enough starter" and I just don't see him bring toughness to the line-up at all. I see Amir slowing down physically, and not at all adding toughness. I've watched him get dismissed too often by many bigs under the boards, and in the paint. Frankly Amir does just enough to be a valued reserve player.
The problem with attaining a star player like Cousins, and George etc. is what do you give up as far as current roster of players. I also like the team we have, I can't see the Celtics without Crowder, Smart, or AB.
Those players, along with Horford are the defensive core of this team.
What I can see is, acquiring a center who can be valuable when the team goes big, with Horford at PF. A few centers around the league fit the bill, guys that will play 21-23 minutes a game, defend, and rebound well... Noel, WCS, Bogut.
Then waiting for another free agent season to get a "name" scorer type to add to the already improving roster.