Complex View with Contextual Filter and Relationships
I have been able to completely solve it. Using a contextual filter, but somehow I had no the options listed in the first edit, so I needed to create the filter first, and then edit again, maybe because I was using a relationship (not very user intuitive, BTW). So here it is important to use the relationship and complete the values below (all outlined in red). Now it works perfectly, the taxonomy terms are listed with the grouped count of items, and filtered by a different taxonomy, with the content Producto joining both taxonomies.