At first, this trade gave me a huge headache, especially with the 4 1st round picks and two swaps. Not to mention close to half of OKC's team coming east and close to half of the Celtics heading west.
The essence of the proposal is to change the core of the team from Tatum, Brown, and Walker to Brown, Collins, and SGA.
SGA is an underrated star in my opinion (20-21: 50% FG%, 40% FG3%, 22.6 PPG). Impressive, but Tatum is just too good to make this trade even though you would be getting Walker off the books.
Including the almost untradeable Walker in the deal is what makes this trade ultimately impossible.
If you were to try to trade Walker by himself right now, maybe the only trade you might be able to make is with a bad team for their equal amount of expiring bad contracts. Because Kemba has two more years on his contract, you would have to probably include 2 1st's or more just to get the other team to consider the deal. Kemba is not an asset right now.
I'm also assuming that you ran this proposal through a trade machine.