I've spent more time on eXaDrums Lite, and there's now an efficient way to communicate between the Raspberry Pi Zero W and my Android Tablet. They both use JSON-RPC, which is both very lightweight and easy to parse.
The app has evolved a bit too, as you can see:
It controls the start and stop button as well as the metronome's tempo.
The next important thing in the development of eXaDrums is the rhythm coach. As I probably already said, it will be greatly improved.
In fact, I'd like to turn it into a proper training tool rather than just a rhythm coach. I have a plan for that, but I'll make sure it's feasible before I spoil you guys.