How to send Struct data from app to ESP32 over BLE?

Yes it is possible. You can send serialized data over BLE. Serializing data means converting your data into binary data (googling serialization will give you more information). You can use any serialization library you want. Choose one that has both Flutter and ESP32 support. I've used PROTOBUF before and I thought it was pretty cool, but it doesn't have ESP32 support. I've used JSON before and it works wonderfully in Flutter, but I'm not sure if there's a ESP32 library.

Let me know if this helped!