The “Hey Siri” trigger is pretty necessary. Imagine having a “Goodnight” scene and saying, “Goodnight” to someone in your home not intending to trigger the scene but it does because the HomePod heard it. Same goes for any scene, for that matter.
It’s not a huge deal, really. You don’t have to pause in between “Hey, Siri” and the command. Just speak naturally.
Use: “Hey, Siri, goodnight”
Don’t use: “Hey, Siri”...[pause, wait for Siri to prompt you]... “Goodnight”