Lightroom not rendering grain properly

Grain and detail sliders will unfortunately always give the wrong results on smart previews since you simply do not have the resolution to render them correctly and applying them at the resolution of the smart preview will give very coarse grain. So best to not even go there (the grain and detail sliders) when working on just smart previews. Other kinds of previews are never used in develop so not much you can do except just working of the actual raw files.
by the way i have not found any significant advantage of using smart previews on recent machines with fast ssd drives. There is potentially a small difference in loading the raw into develop but current ssd drives have made that difference tiny and no longer really relevant at least for my workflow. In fact just the time it takes to generate the smart previews is a time sink (let alone big sink for disk space)  probably much larger than the speed advantage you might get from working with smart previews.

If you are a photographer that works with 1000's of images at a time, by far the fastest workflow is to not generate ANY previews. You import using embedded previews, which gives you almost immediate browsing access in library. Cull and select using that and just develop the keepers from the raw files. Smart previews are only useful in my opinion if you need to work on your files while your originals are offline (e.g. working from a laptop away from the studio where the original files are stored on a NAS server)