I run Blackburn racks front and rear.70% of the weight in front.That's on my 1978 Shogun touring bike,never shimmies.You can ride with no hands screaming down a mountain if you want.

Try to center the front pannier on the axle,neutral balance.If it still shimmies,move the panniers BACK.As you move the weight BACK,it will help hold the front wheel straight.The farther BACK you have the front panniers,the more weight you have to lift when you turn.The more weight you have to lift,the more the wheel wants to stay centered.

Think how much fun a bike would be to ride with a 10ft pole out in front of the front wheel that had a 10# weight on it.Think it will shimmy?Same thing with panniers,just not so extreme.

That's worked for me the last 30 some odd years,for every bike I have setup or helped my friends setup.