We have a male-female ratio of 3 to 4 and have to keep them separated a lot of the time. The complicating factor for us is that the two oldest drakes don't like the younger drake and so they'll team up and beat him up, but they get along fantastically with each other (they're best pals).
Our oldest girl gets along swimmingly with the 2 oldest boys, and they don't really over mate her, so she can hang out with her boyfriends on some days and we let the young drake hang out with his 3 ladies on those days.
The youngest drake has a #1 girlfriend, so if we need to separate him, the little couple can usually go together. He doesn't beat her up very much either, similar to how the older drakes respect the older female.
For your case it might be possible to find similar arrangements, but the drakes can definitely be kept separately. I would just keep an eye on them in case some of the boys start getting bullied--drakes can be brutal to each other even if they're normally sweet birds.
They'll definitely pace and wear down any grass if they can see or hear the girls.
We had more drakes and had a pretty hard time rehoming them. The final one went after having him for almost a year, and even though I knew he was going away eventually, I did get attached to him and it was hard to see him go.
You can probably let them mix together until they're about 4-6 months old and the hormones start kicking in, but after that you'll definitely need to see how things develop.
It's 100% doable, it's just a lot of work (especially if someone needs to get separated out for medical treatment). Just set up separate areas ahead of time while they're not fighting and you should have a lot more options available for how you want to divide them. They also don't have to be strictly separate all the time, they can do some supervised social time depending on how aggressive the boys are.
Sorry for the ramble! I love all my ducks but I'm definitely going to make sure to keep my flock at 1 boy from now on -- 2 at the max if they get along.
edit: also your ducks are super cute!!