I agree with SimonW's comment about Caen over St Malo. I'll accept that St Malo is closer to the western Pyrenees by about 50 miles (under 10% of the likely total distance to be ridden, say an hour to 90 minutes riding time) but you are almost forced to take a potentially less interesting route, running all but parallel to the western seaboard of France. I guess that is why you chose La Rochelle for your single overnight stop?
Caen, though further in distance, lends itself to maybe a more eastern and better north to south route, broadly: Caen, Le Mans, Tours, Perigeuex, Bergerac, Pau.
Go back to your opening post's request:
I'm thinking of doing the ride down through France with the main aim of getting into Spain to explore.
I don't want to bash down the French mways but then again don't want to detour too much as I want to spend more time in Spain.
What's the best scenic route whilst maintaining pace?
You've already saved yourself a chunk of potentially dullish miles by dropping the Cherbourg idea. You won't detour too much (about 50 miles) starting from Caen instead of St Malo. The Caen start point lends itself to a more scenic potential.
Here's an idea for you. Download the ViaMichelin app. Ask it for Caen to say Pau, which is close enough to the western Pyrenees. Put the settings to motorbike, avoid tolls, avoid motorways, all in its 'Discovery' mode. You should see that the software makes two suggestions (a more western and a more eastern route) favouring the more eastern bias. That's broadly the way I'd go, accepting a slightly longer distance by road in exchange for 'better' roads and scenery.