> As long as Spain's judiciary still sees it fit to hand out multi-year sentences to people peacefully following democratic principles, they seem very much an extension of the government and not an independent body.
There's nothing in that paragraph that qualifies as an argument against the separation of powers. In fact, I'd say that's pretty much the opposite, considering that PSOE has been pressuring for lower sentences.
There are such arguments, like how the Constitutional Court judges are elected, but in the end it's up to the Judiciary branch to decide what to do about, and nobody can interfere.