Certainly you can send notes on different MIDI channels. Your NoteON = 144 includes the channel number. If you make some of them 145 that will be Ch2, 146 Ch3 etc.
The delays as you have them means that each note takes a full 1/2 second and they all add up. A better way to do it might be:
MIDImessage(noteON, note5, 100);// Ch1, turn on as many notes as needed
MIDImessage(noteON+1, ...., 100);// Ch2
MIDImessage(noteON+2, ..., 100); // Ch3 etc
delay(300); // then the delay
MIDImessage(noteON, note5, 0);//turn all notes off (note on with velocity 0)
MIDImessage(noteON+1, ...., 0);
MIDImessage(noteON+2, ...., 0);
delay(200); //and then wait 200ms until triggering next notes
That way all notes are sent in about 1/2 second total.
Give that a try.
Steve