A little late to the party, but I have the same gear and experience EXACTLY the same thing and any combination of settings on the wheel or in game do nothing to fix it. As like the O.P. said, you need to grip the wheel TIGHT to hold it steady - loosen the pressure or take one hand off and it's a truggle to hold any car straight.
Video link:
It happens in AC & ACC, but not in F1 20xx or Dirt Rally. So presumably, there's a setting *somewhere* in each game that makes it do this.
Also worth noting that in both Kunos titles (but not present in other games) is this weird "Knocking" feeling at the centre point of the wheel - It kind of clunks over the middle point when changing direction.
I've kinda gotten used to it after owning it for as long as I have, but every time I go to other games, it reminds me that it's not "normal" and frustrates the hell out of me.
Happy to share settings, JSON files and LUT settings if you can point me in the direction of where to locate them, but here's what I have in front of me now.
At the wheel:
Sens: (Varies depending on the car I'm driving, buteither 900, 540 or 360 across different profiles - for ACC, it's 900)
FFB: 70% (although, I can run F120xx or dirt rally at 100% and the issues aren't present)
shck: 100
BR ABS: Off
Drift: -5
Force: 100
Spring: 0 (seems irrelevant to have it on in anything even remotely realistic)
Damper: Off (Same as above)
FEI: 100
MPS FNC: (irrelevant, but its set to) Auto
In game (I change these often, in the hopes it might help, but never seems to - happy to try differen settings if you think these are garbage, but here is what it's currently set as):
Gain:100
Minimum Force: 0 (I read somewhere that this has helped people, but it doesn't change it)
Dynamic Damping: 20%
Road Effects: 30 %
Frequency: 333
Lock: 900
Linearity: 1
Brake Gamma: 1
Gear debouncing: (irrelevant to this, but it's set to) 50ms
Kinda hoping someone has a solution because it's kinda driving me insane and putting a lot of unwanted stress on my wrists/shoulders fighting against it to hold the wheel straight. I understand that you can feel a road surface through the wheel at speed, but there's no way that the video above should be "realistic"