Hi
I had the very same nightmare last year with my young geese. Mine are now coming up a year old and it is now quite easy to tell which are the geese vs ganders.
I have Embdens and I have two girls to one gander.
The geese are MUCH quieter than the gander, they busy themselves eating grass and mooching around. But the gander is very vocal, lots of screeching, extending his neck and trying to look fierce and terrortorial.
He is also quite alot taller and longer necked than the girls and his head is bigger.
I tried vent sexing too, which is quite easy but abit messy. Tip the goose upside down, head behind you as you straddle it and gently prise the vent open, if it is a gander then it will have a little spike/boy part sticking out and obviousley the girls won't.
Hope this helps. You will definately have to cull/sell your extra ganders as they will argue all the time.
xx