No matter what visa you have, you are gonna have to pay duty on musical instruments. We brought over a container of household goods with my non-b and the guitars were the only thing we had to pay duty for.
It took me 3 days and an online application to get an IDP in Australia, hopefully its similarly easy in Spain, it should also be cheap so just do it either way to be safe
Definitely far too unique of a case for general FB advice to be of any value to you. Seek some professional advice as suggested, and honestly if you can't afford that then i would seriously rethink your next steps