Eurorack CV is "common" +/- 5V (bipolar), but can go up to +-12V, inputs should at least tolerate this. I really doubt interfacing this Nanosynth with eurorack is glueless.
Arduino Leonardo is 5V I/O, but modern digital electronics is 3.3V since over a decade.
Further, I wonder how stable it is, can it be tuned accurately, over how many octaves, and how stable is tuning over time? Jumper wires as patch cables, patch cables across the pots, pots without knob, that all makes it cheap, but compromises playability.