Plausible, Fathom, Umami and the others all do cookie-less tracking too. Why don't you add an option to track through cookies? Most serious businesses have the consent for putting analytics cookies there, especially if it's a first-party cookie. This will differentiate you and instantly make this a more serious option for self-hosters who want simple but reliable tracking. Especially if you can set this as a serverside first-party HTTPS cookie.

Alternatively, add an identify() call and let others roll their own solution for this.

Then I would actually trust your retention numbers.

Great stuff though! Impressive launch.