"Day 9 - Andorra (is it worth the drive from Alquezar?)"

IMO no. The drive from Alquezar is a minimum of 4 hours. Add a minimum of 3.5 hours the next day to get to Barcelona and you will have spent 8 hours in 2 days on the road. Andorra in August is a zoo. You'll be lucky if you find a hotel for one night.

Like Andorra, Gavarnie is also a zoo in August, the month when the Spanish and French take their annual vacations. It is unspeakably awful throughout the whole of that month.

Andorra is never worth visiting unless you are going there for the mountains, and that requires more than a fleeting visit. It is a long strip of shops selling cheap-ish consumer goods and booze to crowds of tourists. Since the Pyrenees is a mountain range, there is no need to drive to Andorra along congested, winding roads to see mountains.

"Day 4 - San Sebastian
Day 5 - San Sebastian"
Unless you already have a hotel booked, you can forget that. It is very crowded in August too.

Thanks for the comments! We already have hotel reservations for San Sebastian and Gavarnie. Thanks for the comments on Andorra. We are definitely mountain lovers. It might be worth going somewhere else on the Pyrenees.

I would highly suggest spending the night in Burgos. Nice small city with a Cathedral on the river. Good food and perfect for an after dinner stroll.

I've taken the southern route from San Sebastian to Barcelona.

Going along the northern edge of Spain, spending the night in a lovely town, Jaca, which is on the edge of the Pyrenees.

From there north into the Parc Nacional d'Ordesa y Monte Perdido, which is about 12km from the French border.

Went along the northern edge of the Pyrenees until I dropped down to head to Barcelona.