I can’t recommend the Healtech I linked enough, 5 minute installation and can be configured for multiple functions besides race shifting.
Using the ECU OBD2 plug to feed a phone based light will work, but, you’ll need to set it at least 300-400 RPM below desired shift RPM because the ECU updates slowly.
If you’re going to use the above method, upward pressure on the shifter and let it flat shift when it hits the limited, no need to adjust the Redline. Your fastest times will be shifting at redline in each gear, short shifts will cost you time and lower trap speed.