Read this post and wrote a simple Tampermonkey script as a solution.

// ==UserScript==
// @name         Fix community link
// @version      0.1
// @description  try to take over the world!
// @match        https://sh.itjust.works/post/*
// ==/UserScript==

(function() {
    'use strict';
    const postLinks = document.getElementById("postContent").querySelectorAll("a:not(.community-link)") // get every links that is NOT a community link

    const fixLink = (aTags) => {
        for (let aTag of aTags) {
            const isCommunityLink = aTag.pathname.startsWith("/c/");
            aTag.href = isCommunityLink?aTag.pathname + "@" + aTag.host:aTag.href
        };
    }

    fixLink(postLinks)

    const comments = document.getElementsByClassName("comment-content");

    for (let comment of comments) {
        let commentLinks = comment.querySelectorAll("a:not(.community-link)");
        fixLink(commentLinks)
    }
})();

Any advice? I especially hate the fact that the way to check if it's a link for lemmy community is through pathname but I thought there's can't be a real solution besides listing all the lemmy instances or actually making a request somehow.

Any inputs are welcome!