I love Projectivity Launcher on my Google Streamer, but I can't figure out how to really replace the built-in launcher. Sometimes the device falls back to the default launcher until I press the "home" button on my remote.

Have you tried installing Launcher Manager? https://www.techdoctoruk.com/launcher-manager-for-android-tv...

I had that same issue, and to solve it, I connected to the TV with ADB and disabled the default launcher.