HOW QUITE IS SANTIAGO DE LA LIBERA

Its year round, lots of Spanish families live there year round and its a working town. Yes, it is quieter in winter but its actually really nice outside of July / August when it's at its peak with tourists. Plenty of places open in the months you've said, quietest months are probably January and February when its a lot colder. Its also a good location to explore up and down the coast  so plenty to do depending on your interests.

As for taxis, there are plenty of legal taxis outside the airport or you could look for a local taxi company in Santiago la ribiera and get them to quote you for airport pick up. Cheap taxis are usually illegal ones and not advised. 

Bars, restaurants and some chiringuitos are open all year although obviously busier in June July August and September although you will find Spanish people from the larger cities often visit for the weekends,

As for Private Taxies they are not actually legal in Spain in some of the larger cities and resorts you may find Uber operating, as Dax has said there are plenty of legal taxies outside the airports.