I left my friend’s house and walked up thea gentle slope.

注意:読者が特定の丘のどの斜面について正確に知っていれば、あなたの本来の言い回しは大丈夫です。（ほとんどの読者は、あなたがどこに住んでいるのか、どの斜面について書いているのかを知らないでしょう。）NOTE: If reader know exactly which slope on a given hill, then your original wording is okay. (Most readers probably do not where you live or which slope you are writing about.)

もう1つ可能です: They made me feel theautumn's deepening of autumn.

The westeringly sun was unexpectedly strong, and the countless yellow, brittle leaves had gathered to create a quietly beautiful scene.

ALSO POSSIBLE: The afternoon sun was unexpectedly strong, and the countless yellow, brittle leaves had gathered to create a quietly beautiful scene.

I left my friend’s house and walked up the gentle slope.

The edge of the road was edgcovered with countless leaves, tawny and crisp.

"edged" is more for intentional decoration.

The westersetting sun was unexpectedly strong, and the countless yellow, brittle leaves had gathered to create a quietly beautiful scene.

"westering" is very niche vocabulary (actually, my computer's spell check doesn't believe it's a word, but it does appear to be in the dictionary), usually people talk about "setting sun" instead.

The road was edglined with countless leaves, tawny and crisp.

"Edged" is a bit unnatural here. I would say "lined" instead.

They made me feel the deepeningessence of autumn.

Also a bit unnatural. I used "essence" instead - like the leaves represent what autumn truly feels like.

The westering sun was unexpectedly strong, and the countless yellow, brittle leaves had gathered to create a quietly beautiful scene.

Hmm, you could say "westering sun" but I don't really know anybody in English who uses that terminology. It's super formal, like it's written out of an old book.