be busy Ving (acceptable)
*be busy in Ving (unacceptable)
https://reurl.cc/MjLqNp
https://reurl.cc/ky3Dab
https://forum.wordreference.com/threads/busy-busy-in-preparing.2959063/
https://forum.wordreference.com/threads/busy-in-ing-vs-busy-ing.1620591/
https://forum.wordreference.com/threads/busy-in-ving.3336505/