Although it oncewas fealling as thickly as goose feathers, it melted as soon as it hit the ground. Although it was falling as thickly as goose feathers, it melted as soon as it hit the ground.

I've never heard the comparison of snow to goose feathers and I very much enjoyed it! Typically English speakers will just describe snow as "fluffy" (like sheep wool) or thick and heavy as you also described!

Although it once fell as thickly as goose feathers, it melted as soon as it hit the ground. Although it once fell as thick as goose feathers, it melted as soon as it hit the ground.

It flows better without it being an adverb.

Although it once fell as thickly as goose feathers, it melted as soon as it hit the ground. Although it fell as thickly as goose feathers, it melted as soon as it hit the ground.

"once" + past tense means it happened a long time ago. "It once snowed here" means it snowed here a long time ago.

Although it once fell as thickly as goose feathers, it melted as soon as it hit the ground. Although it fell as thick as goose feathers, it melted as soon as it hit the ground.

Although it {once } fell as thickly as goose feathers, it melted as soon as it hit the ground. Although it {once } fell as thickly as goose feathers, it melted as soon as it hit the ground.

Maybe you can clarify for us what you mean here, because "once" doesn't sound right. Here are two guesses: "at first" "at one time." It does work without the word "once" as others have pointed out. I really like the image of goose feathers too. It's unusual and poetic.

Although iat once fellpoint it was falling as thickly as goose feathers, it melted as soon as it hit the ground. Although at one point it was falling as thickly as goose feathers, it melted as soon as it hit the ground.

When I was on my way home that night, the road was dry, like it never happened. When I was on my way home that night, the road was dry, like it never happened.