Indiana could simply decide that the established tampering penalty (future 2nd round pick and a mild rebuke) is worth it... essentially bake that into the cost of the trade.

But yeah, doing it by the book is tough. Agree in principle on a framework, agree on a draft selection that Phoenix wants but which will still be agreeable to Indiana in case it falls through. They'll also probably need a third team involved to absorb salary, due to base-year compensation rules. Draft the guy, but don't sign him. Then, when free agency hits, Indiana makes the pitch to Ayton, and if he says yes, hope that your third team hasn't changed their mind about absorbing salary.