I remember way back in the late 90s or early 2000s, when Opera was commercial, I bought a lifetime license. I don’t remember the specifics but it was basically a way to support them and it was good for all future versions, forever and ever.
I lost the key long ago and the browser is free now anyway. Still wouldn’t use it.
Make sure you’re also blocking all other outbound DNS at your firewall. Many “smart” devices (and presumably some apps, though I have no way to check this) are hard coded to use 8.8.8.8 regardless of what DCHP says to use.