I did Spanish last year and had to do an IRP. I don’t know much about Spanish myths and legends but it does sound interesting!
I think your best bet is to find a story that particularly interests you - maybe something that a lot of people disagree on. That way you can argue both sides, maybe assess the roots - maybe cultural or religious and draw your own conclusion.
One thing I think is good about this is that it’s super niche, so the likelihood of the examiner knowing about it is low and they won’t probe too deep. This should mean that the questions they ask are more general to your theme and hopefully easier to answer!