As you will need to renew the passport this summer anyway, why not do it before you go? OK you will lose out on a few days but consider the hassle you could be avoiding. What would you do if, for example, the car broke down in Spain and it took 5 days to get the parts to fix it? If one of you were ill and had to stay in hospital for 6 days? The ferry operators go on strike for 7 days and you can't get your sailing?
Maybe a bit far-teched, but my point is, any unexpected delay in returning from Spain could land you in the brown smelly stuff. How annoyed would you be if this happened? Wouldn't you kick yourself for not renewing the passport when you had the chance?