In the long run, I don't see much of a downside to having food in smaller bags. In addition to the advantages you mentioned, it also provides some redundancy, so if an individual mylar bag should fail, or some food be contaminated, it may not affect the entire contents of the bucket.
That said, in the short run you have to weigh the substantial additional trouble against these advantages. 5 1-gallon mylar bags will cost a lot more than 1 5-gallon bag. More oxygen absorbers will probably be used. The amount of work is, let's say, 4 times as much per bucket. Packing a relatively inexpensive staple such as rice you might be able to store 40% more (a guess) for the same cost, which could be a more efficient use of funds if you are not limited by storage space.
Additionally, it is easily possible to cut off just the top of a 5-gallon bag, remove say 20% of the contents, and reseal the top giving some of the same advantages without the higher overhead.