Logseq is already using relative links in Markdown format.
By default, since pages are under /pages, when referencing an asset it will be [](../assets/example.png): those first two dots means “exist from /pages” (because it’s where the Markdown file you are editing is) and then /assets/ means look at the subfolder assets.
But you can move pages and assets freely in your graph folder but you need to update the references accordingly. Logseq still doesn’t provide a built-in file manager that updates references automatically, but for that you can use VS Code or Obsidian: move .md files and assets with their file manager and you should be prompted with a confirmation dialog that ask you if you want to update references accordingly. Logseq will simply respect those changes.
Also take a look at macros for assets management: