The real solution to this would be something like TLS-SRP where you can authenticate both sides of a TLS session with a zero knowledge password proof (devices could ship with a piece of paper containing the generated password, no need for central servers, remote connections to the mothership, gateways, or certificates, or exposing stuff to the internet, or even any internet connectivity at all).

In simple terms it allows you to setup a TLS session by both sides proving they know the secret password, without either side exposing it, thus MITM is unable to capture it, it is an alternative to the cert model that works very well for network local devices.

But of course Chrome and firefox both have zero interest in supporting this use case despite TLS-SRP kicking around for ages now, they'd much rather have you connect to your own devices via a mediated cloud gateway server, for your own safety of course.