As you come to Spain so often, it maybe that you have a friend here in whose name you could buy and insure a car. That way with a written permission from them you could drive without any problems.
Also please note. An international DL is NOT A DL, it is an interpretation of your national DL, thus you must carry both documents when driving.
Insurance in Spain is on the vehicle not on the driver as it is in UK and I believe USA,
Also, the amount of insurance cover very substantially higher in Europe than in USA.
I was astounded how little cover I needed when I had an RV (motorhome) and a sedan (car) in Florida. And I believe that in some States one is not required to have any cover. I opted for much more cover so that in the event of a blameworthy accident the third party could claim reasonable damages.