What would it take to add Bluetooth capability to a digital (musical) keyboard?
The engineer method would be a mux or digital switch or output from the IC8 Synthe Chip on the schematic to a bluetooth audio IC. Proper grounding, decoupled analog and digital power etc.

A hacker way would be to tap the IC11 D/A output at TP4 and TP3 to get the audio before the filter IC, and route that to an internal bluetooth audio module.

But the simple tinkering method is seeing that the piano already provides a method for line out and that line out to bluetooth adapters are cheap and premade. Headphone output would work too. Buying one is the best idea when you already have audio outputs. Or a headphone out to bluetooth adapter that you could even buy at a retail store.

At best you could cut the muting control for the IC19 output inverter (q15 and q14) so that you can mute the internal speakers without muting the line out, as they are tied together. Of course this involves tearing your keyboard apart.

If you want to learn, first step is to find the EP-9 service manual which has the schematic and block diagrams and learn what does what and how. I found the manual with a quick google search.