As long as the pellets aren't too big even fairly young chicks (like 4-6 weeks old) can manage just fine. I only know this because hubby kept putting out layer feed for them when I wasn't looking 🤦♀️, instead of all flock crumble.
At your current rate of feeding you would finish a 100 lb bag in 6 weeks, which assuming the feed is recently milled, would still be very fresh feed. As the younger birds grow up you'll be using up feed faster so being cost efficient is a good idea as long as you don't get so much feed that you...
If you can manage the 100 lb bags then it would probably be the most efficient. I can't even lift 50 lb ones so that would be my restriction, I need to be able to lift it.