Flies are tough. Especially in a small area where you can't move the cattle away from their own feces which is where flies lay eggs. Ivermectin is mainly for internal parasites but I've heard it can help with flies as well. Oilers and dust bags are kind of old fashioned. We used them when I was a kid but I haven't seen anyone using them in decades so I'm guessing they weren't too effective.
What I see people doing these days is using insecticide ear tags or giving them mineral supplements with insect growth regulator (IGR) in it. You can also get mineral with garlic powder which is supposed to help with flies if you want something more natural.
Fly traps and
giant sheets of fly paper by the water tank can help as well I've heard. Moving the herd daily is supposed to help too but obviously that's not going to be an option for you.
If you use the ear tags, you're supposed to rotate the tags you use with different active ingredients or they can develop resistance over time. I've tried a lot of this stuff and it's hard to tell how well any of it works so ymmv. Some individual animals also get flies worse than others for some reason.