bottle rockets and airsoft automatic style guns can work if you are on site.

Otherwise you can get noisemakers like are used in fruit orchards, some go off on a timer others can be triggered remotely (smartphone)

Another effective tool is to buzz bomb them with a RC drone. If you have any teenage neighbor kids you can pay them to randomly stop by and chase the geese around with their drone.

I'm not sure what it is about flashing lights but polished mirrors hanging on string that would flash the sun into their eyes might work.

If you had access to a garden hose under pressure they make a motion activated sprinkler that will hose down anything that is in front of it when the motion detector is tripped. It is meant for deer but should work with anything that doesn't like to get sprayed.

I wonder how it would work if they were bombarded with noise, either the higher pitch sounds (higher than human hearing) or the ultralow sound that you feel more than you hear? There might be some research online about that.