First impressions, etc, but looks pretty good to me 
I’d consider switching out the opto-coupler circuit for one based on a 6N137, which 1) is faster (guaranteed to be within MIDI specs (*)), 2) has an actual logic output instead of a transistor, 3) is more widely available (e.g. Tayda has it), and 4) is cheaper, from what I can tell. The circuit I used for the minimachine is here. But this is optional; the 6N138 is used in a ton of DIY MIDI stuff and people would probably have noticed by now if it was all wonky.
And yeah, I still wonder if we cannot come up with a better 10.6 V supply, but that’s for a different subthread
*) Found some MIDI document that says HP’s 6N138 “have been found acceptable” (which makes me think of Lemongrab, not circuit design) but they don’t seem to make it these days and the non-HP 6N138 datasheets I’ve looked at are less convincing, so may depend on manufacturer. The 6N137 is at least 20× faster.