Hi Garfield, thanks for your reply!
For my piano input I thought I could use a contact mic and feed it through mikrophonie. I know it won't be super high quality but I don't mind it to sound lo-fi.
Regarding audio output I thought I could use an external mixer like a little Behringer one to have both headphones and L/R out, my idea would be to go out from clouds or from A-138b into the mixer, I've read that it can be done and that I don't need to attenuate the signal before feeding it into the external mixer.
Do you think that my idea for managing I/O is doable?