When you have a site with a fixed, tiny amount of users, I'd opt for HTTP basic auth (via HTTPS). Whether you're using nginx, Traefik, Caddy, etc..., it's very easy to setup. If you're using something like Cloudflare Pages, I would guess you could setup a worker to handle it for you (though I'm not familiar enough with workers to be sure).
Lawrence Krauss has given a talk that mentions something similar. He says that we live in a good time because we can still see "everything" around us. At some point in the far future, any observers won't be able to determine many things about the universe because the "stuff" in it will be too far away to observe.
