Yeah that is a putrid trade. heck the Raptors paid more when Kawhi hadn't played in basically a full season.
There is no reasonable way to acquire Leonard without Brown or Tatum plus a boat load of picks and other assets. That said, I might consider trading Brown in that scenario. Brown, Smart, 21, 22, 24, 26 for Leonard. I'd do that. Re-sign Fournier to play SG. Nice little roster with Walker, Fournier, Leonard, Tatum, Williams and still having Thompson, Pritchard, Nesmith, and Langford plus Parker, Semi, etc.