Ah, I see the 6n138 runs on .5 ma of diode current (as opposed to 5ma for the 6n137) and seems to be preferred for midi applications. However since the 220 ohm resistor they recommend will produce 20ma of diode current, the 6n137 should work just as well and since it has an active output shouldn’t need the 74ls14 to drive the repeater circuit to din1. If you like I can modify the circuit to use the 6n137 for you. As well the routing problems are a labeling error in the neopixl strip part. I’m correcting the part and will check it fixes the routing issue and post the corrected sketch with the new part in a bit.
Peter