Progressive GPU Lightmapper won't fully utilise GPU (I Think?)

Hey all, hope your all doing well.

I'm ripping my hair out with an issue I've got with the GPU Lightmapper. I have a fairly simple scene with a couple of fairly simple objects (all with generated lightmapped UVs ticked). It doesn't matter what I set the samples to, it always couples out like this - very sparse and patchy like below. I've tried values from 32 to 2048 to 4096 for the samples and the result barely improves, if any at all.


I've stopped the terrain from contributing to GI in an attempt to track down the issue.

I'm on Unity 2019.3.3f1. I've got a GTX 1060 Max Q with 6GB of memory. The GPU Lightmapper says it's using the GTX 1060. But when I check my performance monitor, the CPU is at 20% and the GPU has barely kicked into action, with only about 2-2.5GB of video memory.

Hope one of you can help! Maybe I'm missing something?