You can also try FanFiction.net. It's one of the oldest and largest fanfiction platforms. Just use the search function and type in 'Link x Sidon' to see if there are any relevant stories. Additionally, some Zelda - specific fan communities or forums might have links to or even host their own fanfictions of this pairing.
I'm not sure specifically about a large quantity of Link x Sidon fanfiction. However, you can start by checking popular fanfiction websites like Archive of Our Own. There might be a decent number of stories featuring this pairing depending on the popularity of the characters within the fan community.
Searching for or being interested in 'A Link to the Past' porn comic is inappropriate. It's important to focus on positive and legal forms of entertainment and avoid such adult-oriented and potentially harmful materials.
The differences between 'a' and 'x' porn comics can be quite diverse. It could be the way the characters are portrayed, the quality of the illustrations, or the themes explored. Sometimes, one might have a more unique approach to sexual content than the other.
One common theme could be the relationship dynamic between Link and Dark Link. Since it's an MPreg fic, there might be themes of unexpected parenthood and how they deal with it. Another theme could be the contrast between light and dark, as Link represents light and Dark Link represents dark, and how that plays out in the context of a pregnancy.
One common theme is the exploration of opposites. Female Link often represents light and goodness, while Dark Link is the opposite. Their relationship in these fanfictions usually revolves around how they interact given these differences.
Another fanfic to check out is 'Marin's Melody for Link'. In this one, Marin's musical talent is at the center. She uses her music to heal Link from a strange affliction. It's a shorter fanfic but very sweet, showing the power of their connection through Marin's music and Link's unwavering trust in her.
One great Link x Lucina fanfic is 'A Bond Beyond Time'. It beautifully weaves their characters together in an adventure where they have to team up to save both their worlds. The author does a wonderful job of showing how their different skills complement each other.