When I have time again I'll try your better solution.

This script might help you. But I didn't test this. Please let me know if you see an issue.
Type and SubType details are available in mycontroller source code

var myImports = new JavaImporter(java.lang, java.util, org.mycontroller.standalone.utils.McUtils, org.mycontroller.standalone.message.IMessage);

with(myImports) {
  // get your gateway via uuid
  var myGateway = mcApi.uidTag().getByUid("my-test-gateway").getResource();
  
  // Import message type
  var Message = Java.type("org.mycontroller.standalone.message.IMessage")
  
  // create message object
  var message = Message.getInstance()
  
  // set gateway id.
  message.setGatewayId(myGateway.id)
  
  // set node EUI
  message.setNodeEui("100")
    
  // set sensor id
  message.setSensorId("200")
  
  // set message type
  message.setType("Request")
  
  // set message sub type
  message.setSubType("Custom")
  
  // set acknowledgement
  message.setAck(0)
  
  // set payload
  message.setPayload("0,4,10")
  
  // send Raw Message
  mcApi.sensor().sendRawMessage(message);
}