Another hand from the book. South was known to have two diamond honours.
6♦ is maybe better but careful play vindicated partner’s choice.
After two rounds of hearts, ♣K, ♦K, ♣Q revealed the bad news.
A diamond ruff to reduce dummy’s trumps revealed the pleasing 5-1 break and set up the trump coup. Press [Next] to run through the play.
Posted inplay hand
Relay slam and trump coup
No point is cashing the spade ace. Just cross to the queen to run diamonds.
Yes, 6D is better, given South showed 3 honours there in your method.