Nanite does support mesh shaders and does use that for the larger triangle rasterization when available. But quad overdraw is brutal with triangles near pixel size, making 4K as expensive to render as 8K, and they've said it's the reason they fall back to compute shaders even when mesh shaders are available for small patches. That might have been part of the reason on Dreams though since there weren't any mesh shaders on PS4.

4K has 8 million pixels so should be able to do pixel per triangle with a separate depth pass and still fall within that 20M even with some overdraw from not all patches able to be culled.

edit: opps thought you said per frame, per ms would give 330M/660M triangles per frame, doesn't seem like a big bottleneck; PS4 was more like 26M per frame though but rendered at 900p-1080p. I think it's translucency was stochastic so lots of stuff could still get culled, but wouldn't be able to cull as aggressively (like nanite with masked materials).