Got the collision detection and resolution working now. Showing in the gif is about ~300 active sliding entities. You can think it as 300 units moving about in this environment. The smoothed voxels are irregularly shaped and thus they require the triangle - box tests. It's a fairly computationally expensive operation given how numerous the triangles are, even after optimizations.
This particular rain effect is unlikely to make it to the final game but its a nice way to detect errors in the terrain collision + resolution system, and to benchmark their performance.
Overall quite happy to get it working, (I thought it would take longer TBH) but still need to make it handle multiple chunks + further optimizations.