There is a very good reason to bag separately even when using food grade buckets. Rodents.
Friend of mine ended up with an infestation of mice. We won’t comment on how or why but the neighbors were to blame.
They chewed through half a dozen buckets and ate their fill over a winter he was out of town. At least if he had them bagged 1 or 5 lb bags, he wouldn’t have lost every grain in each bucket.
And many years back there was a tornado back where I came from. I had left a couple of buckets of supplies for my parents. The pressure from the tornado popped the lids off two buckets. If I had not packed everything in plastic, a lot of stuff may have gotten wet, depending on the circumstances.
And I’d rather unseal a 1lb bag rather than expose a 30lb bucket to air/moisture.