Why Spain lacked a fully domestic dreadnought base

Could Spain have built a full-sized dreadnought entirely from its own industrial base?

Not entirely; Ferrol had the yards, but Spain still had to import armour and related industrial inputs from Britain.

Spain possessed major naval construction facilities at Ferrol. The missing element was a complete domestic heavy industrial chain, especially the ability to make the required armour. Spanish programmes therefore imported essential material and expertise from Britain. A yard capable of assembling a warship does not by itself amount to an independent dreadnought-building system.