I haven’t seen a tail rotor trim yet but you can offset the yaw neutral using the yaw sensitivity curve. That’s how I deal with it in the hover using a TB F1 Flight Stick twist for yaw control. As mentioned above once you get above ~60kts the 407 and MD500 fly pretty straight.
Also, as mentioned above, you’re fighting the Yaw Assist. I found that it almost ignored inputs until about 60% and then suddenly started having a bigger effect. I find it much easier to control the yaw without the assist.