I tried it for the first time yesterday and had the same problem on the T500RS.
First and foremost, I'm 90% certain the wheel has the latest firmware installed as I check for updates every now and then (will check this later though), and the last time I did it was a couple of months ago.
After the disastrous first run where the FFB did the non-stop swerving, I started messing around with the wheel settings and what seemed to work better for me was to raise the Dynamic Dampening a little bit, to 120-130, which created a sort of deadzone effect, and then dropped the gain to 40 or so. What this does, is it stops the wheel from trying to swerve from left to right to left during the straights, but the downsides are that 1) it removes a lot of feel from the wheel, it almost feels like your hands are numb when driving, there's not a lot of feel for what's going on with the car and 2) during corners instead of feeling like the car wants you to go somewhere else, the FFB just suddenly drops sometimes as if instead of swerving it's just letting go.
It turns that mess into a driveable setup, but it's almost impossible to try a go for a fast lap because the FFBs unpredictability along corners and it's sudden lack of strenght during them (due to the very weakened swerve nature of the FFB) and specially when dealing with oversteer... It's guaranteed you'll either be facing the wall or the wrong way in no time if you try to push.
I also tried the Mode+Right and set it to 540º of Rotation, but it didn't seem to make much difference.
I did a bit of research and found very few things related to this specific problem, but it seems like the T500RS is giving a lot a people headaches with very different problems, so it seems at least us T500RS drivers will, unfortunately, have to wait for the inevitable patch or update to correct this.