Yeah you’re right, that is too young for maintenance, they still need to be on a grower. Purina flock raiser is a decent grower, it can be used as an all flock/maintenance feed when they’re adults too, hence why mine are still on it.
Later you can reduce it to something around 17% protein between laying/molting seasons when they’re adults and give them the flock raiser or an all flock at 20% protein when they are molting, laying, or in colder months if you feel the need.
You can try putting them on a meatbird type feed to get the thin one’s weight up, something between 22% and 30% protein. Purina game bird chow is 30%, so is encore avian gro rite.
https://www.tractorsupply.com/tsc/product/purina-game-bird-startena-bmd-50-medicated-50-lb
https://www.tractorsupply.com/tsc/product/encore-avian-gro-rite-for-chickens-2-lb-54010
I used encore as tube feeding formula once when I was in a pinch when my gander was sick and the post office kept losing his normal formula. He did NOT like the taste of encore though so you might have better luck with the game bird chow. Don’t worry about it being medicated, it won’t hurt them.
30% protein isn’t healthy long term, it’s only to be used to put weight on in times of stress or if you’re raising meat birds though, so it shouldn’t be used as a maintenance.
You can also supplement with treats like soldier fly larva and mixing some of their feed with water to make a soupy mash and add a little buttermilk. Buttermilk is an excellent probiotic that’s packed with vitamins and protein and it’s low in lactose so it won’t upset their stomachs.
You might also consider getting a fecal stain or float on the thin goose, their poor weight could be de to an underlying issue like parasites or some sort of illness they may be fighting.
You probably will get some issues switching feed at some point, geese are notoriously picky and suspicious of new foods, you can try mixing the new feed with the old for awhile to get them used to it, you can also mix it with water to make the soup, geese love foraging in water, and it will get them to at least try it and get used to it.