With fast moving collision geometry and FLIP, what are some things to try to stop fluid from passing thru your geometry?

Things I've tried
- increasing substeps(using 8 currently)
- reducing particle separation (I'm at .01 currently)

What else should I try?

Try to increase exterior voxel band for your collision geo.

Make sure your geo is not a flat plane, check your vdb of your collision geo to see what the simulation is actually seeing.
If your collision geo VDB doesn't have the detail you want the water to collide with then decrease voxel size. Make sure voxel size is consistent throughout simulation.

If your scene scale is too small then it won't matter the resolution you have, the geometry will go through. Your geo should collide at 2 substeps so go ahead and turn that back down.
Scene scale matters in dynamics and FLIP isn't exactly meant for small water sims. It can be done though.

Hope this helps!