I haven’t used the Sketchfab viewer in VR. It is probably more lightweight than a whole Unreal Engine application, but a heavy model could still affect performance. High poly counts and high draw calls (material IDs and separate meshes) could still be problematic. I think you just need to test that.
If your entire model can reasonably be viewed from one position, I think it’s generally best to combine as much as possible. In my experience, draw calls are much more impactful than polygons these days. It’s true that you can leverage culling to not render the meshes that are behind other meshes or behind the camera, but most Solidworks models I’ve seen have been somewhat small in size and have a lot of pieces. If it was a city-scale CAD model, I’m less likely to suggest merging it all together. But something the size of a motorcycle or computer, I would merge anything that doesn’t need to explicitly move around separately.
Nanite is exclusive to UE and exclusive to the supported platforms. I don’t think standalone VR apps (Android) support it yet, but check the latest release notes. Sketchfab will not have Nanite. For the most part, if you can enable Nanite, then you should. But always test a before and after to check the performance results.