Giving up Smart, Brown, and both BRK picks is too much considering
(a) Cousins is a troubled asset given his off-court issues;
(b) He has one year on his contract after this one;
(c) He is likely to test FA;
(d) Our coach probably isn't keen on coaching him.
Cousins is a great talent, but I would be very uneasy about completely emptying the cupboard for him.
My goal in any Cousins trade discussion would be to keep one of the BRK picks, one of Bradley and Crowder, and one of Smart and Brown. You obviously have to give up a lot of pieces to trade for a talent like Cousins. But you can't give up everything.
The Celts need to hold onto IT, AB/Crowder, Horford, and at least a couple younger assets to keep the cupboard stocked for the next few years.
And honestly, I think you would have to be very careful trading away either of AB and Crowder, considering how crucial those guys are to the current culture of the team.
I would try to structure a deal along the lines of:
Cousins + Casspi
for
Amir + Brown + Rozier + BRK (17 OR 18) + Non-BRK 1st
That's a top 3 prospect, a 1st round prospect, a veteran expiring role player, a likely top 5 pick, and another 1st round pick.