The best guardian dog is an actual guardian dog such as Pyrenean Mountain Dog or Anatolian Karabash. We had a Karabash x and she was great with sheep, poultry and pigs. Ours didn't live outside though as she wasn't trained from birth. Major drawbacks are that they are expensive and BIG.
We have two terriers, one a Patterdale x Cairn - wonderful ratter but decidedly glaiket where foxes are concerned. She totally ignores the poultry other than to steal eggs, and is mostly indifferent to sheep, except when they have new lambs when she's terrified of them. The geese ignore her.
The other is a Jack Russel cross - great at digging out mice, rabbits, moles etc, but again hopeless with foxes. Having said that he has gone for a fighting dog which was attacking the other dog and he was well on the way to killing it

so not a coward. He's the one who helps round up the sheep. Our new geese hate him, and one of the hens does the full kangaroo-style ripping out his guts the way two hens fight, but he just ignores them all and gets on with stealing grain or digging pits to break your ankle in.
I think any dog which is going to be left alone all day and every day to guard poultry needs to be a special dog and happy with its own company - not great welfare for any dog. Can't you just improve your fencing?