The UHD Blu-ray is unfortunately plagued with frozen grain all over the place, so there’s no chance of really fixing that short of scrubbing whatever grain is left and regraining and that would destroy even more fine detail.
Would there be any benefit to adding light fake grain without first scrubbing the frozen grain? Just enough to mask the frozen grain a little bit?
I should add, I don’t know enough about how these things work to know what’s possible/what looks decent.