Generally the tax office doesn't take ignorance as an excuse
I'm not so sure. Sometimes you may find that you know a bit more than the tax office and they can be very nice and helpful. I agree that once you go down the declaration route you cannot very well stop but you can't just do nothing. Suppose Podemos win the election in November and take even more draconian action against asset owners who have not declared? Bite the bullet, plead ignorance but as one now seeking help and enlightenment. Good luck and let us know how this all turns out for you.