I see a couple of problems with your initial plan.
If you put the food directly into a bucket, displace the O2 with CO2 and put on a lid, there is no way to tell if you successfully displaced all the O2.
Another problem is that the plastic is permeable.
Using a Mylar bag solves this problem, you can see that the O2 has been removed as the Mylar sucks down,
and you can always open the buckets periodically and check them.
If the Mylar is no longer sucked down, then you have lost your seal.
Gamma lids are a good item to have to put on a 5gal bucket AFTER it is opened and you begin using the food.
Their purpose is to allow you easy access to your food.
You don't need easy access to food that your are storing long term.
We use a gamma lid on a 6gal food grade bucket to store sugar in our pantry.
We re-fill our inside canister when needed from this bucket.
Yes you can remove and replace the regular lid, but the gamma lid makes life easier.
As for sealing, I use a clothes iron and a metal level.
This thread is a pretty good How-To for storing in Mylar.