As for goose care, there isn’t a lot of information out there because there isn’t a lot of goose owners compared to chicken owners, so I’m not surprised you had trouble, I did too when I first started.
On top of that there’s a lot of BAD information, Facebook and feed store employees are common sources of that.
Anyone on backyard chickens can fall victim to repeating misinformation, I have, but there are a lot of more informed poultry owners here that can correct or give better advice.
So some basic goose care if it helps:
Goslings and geese have higher nutrient needs than chickens and chicks. Goslings and geese need more of all of the Bs along with E, A, and everything else.
B vitamins and vitamin C are water soluble so there isn’t a risk with overdosing, other vitamins like E, A, D, are not water soluble so don’t go overboard on them.
(Your goose could probably use a good dose of everything for awhile though.)
Medicated feed isn’t actually bad for waterfowl, but most medicated feed is formulated for chicks so it’s too low in everything for waterfowl. People thought that the symptoms of vitamin deficiency that they were seeing was the effects of poisoning from the coccidia medication, but it was vitamin deficiency. The medication isn’t actualy toxic to waterfowl....unless you’re giving denegard (tiamulin) at the same time, the interaction is toxic to everything....
Small breeds and large breeds can have different needs. For large breeds 20/ 22% protein feed given year round is fine, for small breeds that can be too much and they can have obesity issues.
Geese have higher nutrition needs when molting, 22% protein is just fine again.
Adults that aren’t molting can be fed a 17% protein feed in the summer, especially small breeds, larger breeds might need more or might not, it can also depend on the individual and how much pasture they have.
Laying females need calcium supplements, egg shell, oystershell, or a layer feed, non laying females, goslings, and ganders should not be given layer feed so offering shells is preferable if you have a mixed flock.
If you have harsh winters 30% protein can be given on very cold nights, otherwise 20/ 22% is needed in winter months.
If you’re mixing scratch with feed, it depleats the nutrient intake of the feed, they’ll be filling up with scratch instead of what they need, scratch is a fun treat that they love, but it should be given as a treat. Corn doesn’t have a lot of nutritional value but it can help keep them warmer on cold nights in winter, just something to consider.
As for illnesses, there are some goose specific diseases but they’re generally rare, geese can catch a lot of things chickens can but their symptoms can differ at times which makes diagnosis tricky. There are some illnesses they are immune too, or at least thought to be immune to, like leukosis, mereks, and Newcastle for example.
Other things like mycoplasma G and mycoplasma S they can get, a lot of the Veterinary world seems to not be fully aware of this among other things though.
Companion animal (cats, dogs, and to some extent horses) medicine is decades behind human medicine. Avian/exotic medicine is behind companion animal medicine, and wishing exotic medicine what medically is known for parrots and other high value birds is ahead of “poultry.”
Chicken medical knowledge would have been fairly poor if it wasn’t for the growing populartpity of backyard chicken ownership, but because of that most studies for poultry are about chickens, there are nearly none for geese.
So what I myself do when I have some medical issue for my geese and I can’t find any information about it is I look up chicken medical articles and websites. I have to weed through some of the conditions that geese can’t get or are unlikely to though.
If chicken medicine has failed me I go to parrot medicine, which is usually pretty good information, because a vet might not care much about “poultry” but they’re going to be extra careful when they have a $$$$ endangered macaw as their patient.
When avian medicine has failed me human medal information is where I go next.
Not everything is relevant across species but the amount of things that are the same is astounding and and it can be a huge help.
If you know a farm vet or an avian vet that sees poultry you’re lucky, there aren’t a lot of either anymore.