Your geese and property are beautiful. I agree that with your lush green pasture in Ireland, you would not need to do much to supplement their diet. The grains you provide are probably sufficient, but I also view bread and white rice as filler treats. As for your gander's lean condition and conjunctivitis, it could be anything.... viral, bacterial, fungal or parasitic and each would require different treatment. Try googling "waterfowl conjunctivitis" for more information. I would (at a minimum) catch him, give him a careful exam, and keep him isolated until you can figure out what is wrong. I assume you cannot take him to a local vet for exam/treatment?