Yeah that a bigger discussion around 4k masters. Usually I’ve found if the Blu-ray has a lot of grain it’s likely a newer scan more true to the film print. Some older ones were just upscaled from some older scan and some they do cleanup - some is fine like removing defects not in the print. Definitely a balancing act. If it’s real film grain I’m fine with it even if it’s excessive as long as it’s not added in for the sake of making an upscaled copy look more legit.

There used to be a site that tracked which copies were real but seems to have been abandoned.

Remux all the way. If the movie is grainy that’s just how it is. I understand some people like smoothing but I’d rather the full quality.