Yes, and not trying to be mean in any of this its just most don't look too far ahead, but you say you won't trim your aggressive male a very temp thing, because it want him breeding your females. breeding leads to eggs, that you say you don't eat the eggs or extra ducks, can't and don't want to rehome any, and you get new ones.. you have no space you say to temp even seperate your male that's terrified and attacked, and your trying to breed them which leads to ducklings that need more space and can't be cramned together in small pen with all adults together with them, them nowhere to get far enough away to be even feel safe from other females and aggressive drake. all these statements are now conflicting to what would leave me to think your duck numbers are going up.
You can try gently but firmly forcing dominant drake to submit much like with dogs, trying to build confidence of brother beat on, but space trouble will conflict over onto your females eventually too. a blood feather torn out wrong can kill a duck in minutes supposedly, especially if this duck is weakened by not eating (check for full crop and feel for bone on chest, you should feel feed in crop on bottom of neck, and shouldmt feel bone on chest).
i cut down from seven to four males recently, roaming on nine to twelve acres (only three constantly foraged and grazed on), and all is greatly improved. knowing that do you think two males will suddenly get along now that not usedto constantly being penned, when they used to best bros? i actually had to kill two of meanest in this free range (no fences at all) situation they were raised in with all but two recently added new females that attack them, otherwise nothing changing and these two were very aggressively hurting females and males etc. so now think that two of, how many males to balance scales out, grown up free range, with probly many more females all been together to work out pecking order from ducklings, and had older ducks and parents to put in line when got too bad, and put them in small area I'm sure they can see out, with new females as well I'm sure, and they will have problems. if they didn't work it out in a week to month, sadly it may not happen, but then interfering in initial fighting may make/made worse trouble. i coddled Ferdinand and now he got soft, but he stillis preferred breeder to ladies as lets them eat brings them feed, protective of them and young that not even same species, etc. one original mean drake defends female came with from preds too, but was aggressive father of known young of his he was only other duck around since female laid.