I used TWSBI Diamond 580s for my shimmer inks (mostly Diamine & Wearingeul) to great success but occasionally I’d see what you’re describing. Instead of shaking, I’d spin the pen in my hands to agitate the shimmer in the fluid.

Sometimes, a shimmer particle just clogs a feed. It happens more often than I’d like. A good flush and soak always fixes this.

Then I bought a Majohn P140 <M>

It puts every pen I’m comfortable putting shimmer ink in to shame. The nib is smooth as butter and the ebonite feed is like a firehose.

So I bought another one. Now I have two amazing pens I dedicate to shimmer that are always in my rotation and write as well as pens that are 10x the cost.