My local tourist office provided the following in response to my question about where to list in order to appeal to the domestic market which is so buoyant right now. I hope it's useful. She's narrowed to Andalusia where I am, but they are for Spain in general.
https://www.escapadarural.com/casas-rurales/andalucia
https://www.clubrural.com/casas-rurales/andalucia
https://www.sensacionrural.es/
https://www.turismorural.com/
http://www.andaluciaturismorural.com/
https://www.rusticae.es
https://turismoruralenandalucia.com/
https://www.casasrurales.net/casas-rurales/andalucia
https://www.ruralidays.com/casas-rurales/andalucia/
https://www.ruralzoom.com/turismo-rural-en-andalucia/
https://www.holidu.es
https://www.hometogo.es/
https://oasishunters.com/
https://www.casamundo.es/