Multi-team deal that doesn't include Castle:
Sac Sends: Fox, Huerter, Lyles
Sac Receives: Vassel, Sochan, John Collins, 2025 Spurs pick, 2025 Hawks pick, 2027 Hawks pick
Utah Sends: Collins
Utah Receives: Huerter and Lyles, and Bulls protected pick from Spurs
Spurs Send: Vassel, Sochan, and 2 Hawks picks, 2025 spurs pick, and Bulls protected pick
Spurs Receive: Fox
That works salary wise, though the issue would be in the picks. You could do a lot worse. Spurs probably balk at the 4 picks. But Kings would be set up nicely.
Domas (36) | Collins (12)
Keegan (16) | Sochan (22) | Collins (10)
Demar (32) | Keegan (16)
Keon (20) | Vassel (28)
Monk (32) | Keon (8) | Carter (8)
Domas 36
Monk, Demar, Keegan - 32
Keon, Vassel - 28
Collins and Sochan - 22
Carter - 8
That is a way bigger and more defensive team. You add playable size to to 4/5 rotation (Collins), get to flex Keegan down to the 3 some, add a really strong defender (Sochan), and add an interesting wing piece in Vassel.
Then you trade Demar in the offseason, and either groom Vassel as your future 3, or use the assets from the Demar trade (and the three draft picks) to get a Demar replacement at the 3, with Vassel playing a super 6th man role.