That's a problem here to, more mm would be nice, new 800 or 1200 5.6? (one can only dream...)
That's where the 800/11 comes in. A pretty darn capable lens. With some restrictions, yes. But capable. I have one although I haven't used it much since getting the 100-500.
I shoot a lot in Waterland and they're everywhere. I can usually get pretty close. Or wait until they fly by. Their numbers too have declined though. They're amazingly good flyers and the males show off how good they are at it. It's an amazing spectacle. A couple of them will go up and do their shenanigans. They will even fly upside down. I took some shots of a couple earlier this year. They were a bit too far away for anything good, and light was difficult but here are two shots. Pretty big crops though.