One common theme could be the exploration of their relationship dynamics. It might involve how they interact, whether it's in a friendly way or with some underlying tension.
I'm not sure about specific 'once ler x greed ler fanfic' off the top of my head. You could try searching on popular fanfic websites like Archive of Our Own. They usually have a wide variety of fanfictions in different categories.
The key to reading manhwas is to immerse yourself in the story. Pay attention to the details in the art and the expressions of the characters. You could also join online communities or forums to discuss and get different perspectives on the manhwas you're reading.
You can try some adult-oriented comic websites. But be aware that accessing such content may have legal and ethical considerations depending on your region.
Some popular platforms to read manhwas are Lezhin Comics and Tapas. Both offer various genres and stories to choose from. Also, you might find some on Naver Webtoon, which has a large user base and a diverse range of titles.
You might find manhwas on apps specifically designed for comic lovers. Check out apps like Tappytoon. Also, some libraries might have digital collections of manhwas that you can access with a membership.