St Malo to Santander

that is what we chose to do.
there is a section of hardpack before Ploermel, then you rejoin the Nantes/Brest canal path which is also hardpack toepath. You could chose to go through Redon or cross the (high-up) bridge in la Roche St-Bernard.
i would try to go through the salt marshes around Guerande, very pretty.

Nice one. Hardpack/towpath fine, I reckon. My routing website has taken me over the Barrage d'Arzal instead of La Roche St-Bernard and skirting round Guerande, though I can easily re-route to take in the salt marshes. That said, I'll be seeing a fair few of those on my way south from La Baule/Pornichet towards Noirmoutier.

Yes they are not an uncommon sight along the western coast. If you have been around those parts already no bother, just that Guérande has a bit of that posh "new forest demographic" vibe so it's just a nice to cycle through if it happens. Only mentionning it.

Well, I did ask for tips...

Honestly, if it's worth the slight detour, I can totally take it in. It's not as long a day in the saddle as the previous day, so looping round the other side of Guérande shouldn't be an issue. Plus, I could easily stay there instead of the more commercial-looking La Baule/Pornichet.

I did the ride in 2017 and one of the highlights was the rather brutalist Notre-Dame Church of Royan

https://www.instagram.com/p/BZLPCuvAvbf/?img_index=1

to be fair given the coastline, everything is a detour from brittany to vendée.

Le croisic is nice, if you can be arsed to loop around the small peninsula, there is a natural pool at the tip. From Guerande you could follow the route to Batz, loop and return via the south coast to La Baule.
La Baule is more commercial but it's not to bad, some of the architecture is enjoyable.

Batz looks cool and has a brewery. Might stay there.

I’ve heard there is a shuttle service for cyclists over the bridge.

It is cool. the peninsula is overbuilt in general but obnoxious, and has a nice laid back feel if travelling not in peak season.

re : bridge comment below, there is a shuttle service, which was a 6 or 8 seater renault with a trailer. we waited 1 1/2 hours to take it ( as were touring with a kids bike seat ), more cyclists arrived and could not get in. I'd definitly not wait if riding solo and in decent weather

We rode the bridge, it was much hairier on the way down than up!!