How do I share my Thunderbolt Ethernet through WiFi in a hidden way?
While most routers provide the option to hide your SSID by not broadcasting it, to the best of my knowledge there is no built-in macOS way to do the same thing. 

However, if the main reason you wanted to hide it was because you wanted to limit access to only those devices you wanted to connect to it, you can do this by setting a password on the Wi-Fi network your Mac is creating/sharing.

You can do this from the System Preferences > Sharing pane. 

When you look at the Internet Sharing options, click on "Wi-Fi Options" button at the bottom-right of the screen and then you will be able to set the name, channel, and password of your ad-hoc network.

By doing this, other devices may be able to see your network, but they can't connect without the password.