I went in September last year (9th-12th September, for reference), and did not find Express an essential by any means. On the two full park days I had, I got 12 rides in on each, and on my first day, I did all coasters bar Tami-Tami, as well as rerides on Shambhala and a ride on two of the water rides, without buying Express.
I can’t vouch for what it’s like in proper peak season, but when I went, I found the park perfectly manageable without Express. And I should also add that one day of my visit fell during a Catalonian public holiday, which was absolutely fine.