Hola and welcome!
I would love to make my trip at least 4 weeks long so thought I’d then head west to Barcelona.
I am absolutely the last person to be commenting on a sense of direction
but to clarify.... Barcelona is south of Montpellier, no? You don't mean Bilbao do you?
However there isn’t too much information on the route between Montpellier and Barcelona and the info I have found doesn’t sound overly positive.
My vague recollection from general reading is that that part of the coast can be very congested, and my experience backs that up. However, there is no reason to stick with the coast.
Even though EuroVelo says this is a developed route
I've learned a long time ago to take EV routes with a pinch of salt
Some are excellent and some are........ in development. And there's a lot up to personal taste.
In fairness, I believe the EV site now has gpx files of completed sections.
When CT has suggested an alternative option to an EV route I nearly always take it and don't regret it.
Anyone had any experience of this stretch?
I have some from south of the border down to Barcelona and around the general area. I loved it but my visits to the coast were short and not terribly sweet. Incredibly busy. Lots of stressed out tourists. And some crazy rates to pitch a tent. Up to €50 and little opportunity to wild camp (it's illegal in Spain, tolerated mostly in some areas but didn't feel comfortable here).
However, away from the Crazy Coast there are some wonderful roads through stunning (and hilly!) country with little tourist traffic and plenty of respectful drivers.
I should add that having covered a pretty large part of Spain Catalonia was the only place that felt a bit "off". In one small town, stopped for a bite of lunch several people warned me not to leave my old, battered and heavily loaded bike unattended. In other places I happily left it unlocked and out of sight as I wandered. Not in Catalonia.
cycle up the West Coast of France
I did it in the opposite direction and I really enjoyed it but.... I love the sea and put me beside water and I'm a happy bunny. I've read of many people who found it boring. Tastes differ. Expensive camping too in the south of France.
I use
https://cycle.travel/map as my goto planner and it very rarely lets me down, picking quiet roads and bike paths. If you or your bike are finnicky about surfaces you may want to plan a little deeper than I tend to do.
I am probably not best placed to advise because I tend not to plan anything beyond a very loose destination a week or two down the road and that often changes. I'm also not much of a follower of bike routes - I pick a place I want to go and ride there and to be fair to Spain and Catalonia it's a place where that is entirely possible. I was there in July/August so slap bang in the tourist madness, too.
CT shows a route along the French side of the Pyrenees to the French coast which might be worth investigating. I recall reading of a gravel type route, remote in parts, that ran along the Spanish foothills also to the coast. Can't remember where I came across it but it sounded wonderful. Tough, but wonderful. Bikepacking.com might have more info.
You may want to check out
https://www.crazyguyonabike.com/maps/?o=3d2 a site for bike touring Journals. That's the map page - you can zoom in on likely places, see what journals are from that area then access the journals. Great info, wonderful inspiration, just stay away from the forums.
I'd imagine that logistics of getting home might play a part in choosing a route?
As for is it worth it? That's up to you.
Good luck!