You can add symlinks in your assets folder that point to an external folder. You may change the folder location but just update the symlink and Logseq references will keep working.
If you need to reorganize the subfolders in your asset folder you can use another app like VS Code or Obsidian that updates references in Markdown files when you move assets.
Logseq with version 0.9.0 will let you “mount” external folders so you won’t even need manual symlinks.