Lemmy-ui supports SEO, and also has opengraph tags. If there's anything else needs to be added, we're open to PRs.
Side note: For me personally, as @FrostySpectacles@lemmy.ml suggested, SEO shouldn't be a focus. SEO is such a gamed system, catering to a few giant search companies, and results are increasingly becoming unusable, especially in the past few years. I can barely find the things I want to search for, and almost always have better luck using internal sites search engines. So I'd rather focus on improving lemmy's search capabalities and filtering, than catering to google.