If you want the best quality AV1 encode while keeping as much of the “original” grain as possible, you can opt to use these options at CRF below 20.
I have tested these options with my main problem sources, Alien and Alien 3 and it looks very good at preset 4, crf 18. It will look better than x265 at same file sizes.
Disabling Temporal Filtering is a must as it messes with the grain too much, you can see this when doing frame by frame seeking.
tune=0:enable-overlays=1:enable-tf=0:enable-variance-boost=1