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I had originally planned to cycle this
That looks brutal.
Also it does get very hot down on the plain, with not much cover and not a lot of opportunity for stopping for refreshments either. It's a lot nicer for scenic villages and vineyards and food shops up the Alsace wine route. On the German side you can go through the Black Forest but you end up further from the river and it's harder to pick a sensible route without going a long way out of your way.
I cycled from the Scottish Borders to Basel this summer, via the Hull ferry.
That section of the Rhine is honestly the worst - I ended up covering the whole section in a day as I wanted to get it over with. It's often very exposed which is miserable if it's hot, and you are often on raised gravel paths with either boring woods on both sides, or woods and the Rhine.
The only section I found interesting was this bit. I cycled on the German side of the Rhine, I'm not sure the French side is any different (just read the comment above, seems not!).
Also it's worth noting that there were a number of construction projects along that section, which involved a number of badly sign-posted diversions. I had originally planned to cycle this had I managed to get my bike more off-road worthy by the time I started my trip.
TLDR that's the worst bit, honestly, don't bother. The section between Hoek/Rotterdam and Koblenz was the best bit!