I’m not sure this will ever work with the digitalWrite command. That’s a command that’s part of the arduino library and the user has no control when and how quickly it’s executed. You are probably better off using the TX of your uart module connected to your IR transmitter. You will need a driver of sorts to drive the LEDs but it’ll work. Baud-rate determines timing between bits and you can adjust timing between bytes with a timer or counter.

What is the application?