Is there any way to adapt the steering system to reduce its sensitivity when steering at speeds above 100 km/h?
Compared to, for example, German cars, in the DS7 you have to constantly correct your driving with the steering wheel.
That’s what you got the Lane Assist for. 
But no, I haven’t really noticed the issue (and I had 2 Mercs in succession right before the DS7).
Sure you don’t have too much pressure in the tires, or anything like that?
Alignment might be off… Or different/wrong pressure in the tyres…
Pressure is fine.
Lane assist is great, but I’m still aware enough to drive myself and don’t want to give that pleasure to a machine.
I saw a few reviews about the DS7 on YT with a similar impression but I didn’t think it would be so annoying.
Tracking could be out, tyre pressures wrong or not equal, shock absorber issue.
I’d start with the most common and easiest/cheapest culprits, tyre pressures, then get your tracking looked at.
Pressures checked and is fine, no changes.