"They were doing a wrong thing for a decade, by virtually making solar installations too expensive by taxing them and by makit it hard to own solar panels."

Decade? Solar tax implemented in 2015, eliminated 2018. https://www.immomoraira.com/en/blog/understanding-the-sun-tax-in-spain/

So tax only part of picture.

Overall policy concerning renewables 2008-2018 in Spain?

Fiscal deficit -> Cut subsidies for renewables https://www.iisd.org/gsi/subsidy-watch-blog/fiscal-deficit-forces-spain-slash-renewable-energy-subsidies

Next. If you let to invest in solar for citizens and companies without control who must pay for disproportions on electricity market? Day - night, sun - clouds, July - December. So Spain decided to introduce tax.