You did a really well job.
It's a cool hardware, really fun to use and not the least, it is power by the gameboy

Interesting side note: I originally created Arduinoboy because I was having issues with my LSDJMC2.

I decided to go to my basement and to bring back my arduinoboy to double check it. I'm not able to reproduce this behaviour with lsdj 3.7.4 and nanoloop 1.7.5.
The sync seem perfect to my hear.
Can't link it to nanoloop 2.7.x because I don't have the cable to do that.
Don't know the firmware of the arduinoboy, but it must be 3 years old.
It works as fine as my lsdjmc2, so everything is perfect

Can't remember what exact setup I use three years ago, but I remember buying a lsdjmc2 for fixing this sync issue.

5ms is within a normal range. Plus and minus buttons will not solve that issue, since they essentially either skip a MIDI tick, or add a MIDI tick. A single MIDI tick at 120 BPM is about 20.84 MS, which means you'll either add 20ms or subtract 20ms by hitting those buttons.

I'd like to think the same, because arduino is fun BUT.
When I sync nanoloop 2 in midi slave, lsdjmc2 is able to adjust the midi sync signal with + - button to be in perfect sync, and it works nicely with nanoloop 2.x.
For an unknow reason, for me, nanoloop midi sync is not so easy. The pulse sync seem to be better, but sometime i need midi.

Arduinoboy was something like 5ms out of sync in midi.... Just something really boring.
And there was nothing to workaround it.
And here I talk about multiple hardware gears which talk midi without issue with a good 8x8 midi switch.
Maybe it is a firmware issue, maybe a nanoloop 2.x issue with my arduinoboy and I need to update it... I talk about something like 3 years ago. but, I don't like to be unable to fix this midi sync issue with a + - button.

So for me lsdjmc2 works better for my use.
And with a good case, and good button the two cost nearly the same.
Think there is no issue with lsdj, but it's not my preffer sequencer/synth.

Just get an arduinoboy they are superior in just about every way,

If you have one and want to sell it.
Tell me. It cost 100 euros new, but if I can have one for less, I'm in.