KITAS HOLDER TRAVELLING ABROAD

I guess you can stay out of the country as long as you like as the Kitas allows you work here legally nothing else.
However if you wish to renew it then you would need to give at least 2 months for the process to begin again. But your employer would tell you more and might not be overjoyed if you are out of the country for a long time considering the amount of effort taken to get you a Kitas, unless they are sending you to Turkey.
Pianos not my forte I'm afraid but I am sure there will be hassle.

When you consider shipping costs, the possibility of damage in transit and messing around with customs, it might well be easier to sell it and buy a new (or second hand one) here.
Loads of music shops to choose from.

The MERP and KITAS.
You can stay outside the country as long as both are valid, and re-enter as many times as you wish within the same time period, but immigration are likely to take a closer look at you when you re-apply.

Thank you for your prompt reply