The train is ok. We had a cabin for 4 and that way you dont have problems with your luggage. An other option would be to hire a car. With the big companies this is possible that you pick it up in france and drop it off in Spain. There of course will be a premium for that but it can be done.
Easyjet are very expensive for extra luggage. As you know the dates you are travelling try
www.iberia.com as soon as the flights become available( about 10 monthes before) They often have some cheap seats when the flights are first released. Tey can be more flexible about luggage as the flights are filled with business travellers.