I have this problem too. I do not want to have to turn off Wi-Fi calling on my two iPads, but as the OP related, Silence Unknown Callers only works on the iPhone, whereas the spam calls still ring on the other devices. It’s extremely annoying. If there’s an iPad app that can apply a Band-Aid, that would be helpful, but Apple can and should fix it themselves. (I’ve seen complaints on Apple support forums about the issue, and the company has confirmed that so far, there’s no way to silence unknown callers on the iPad. It’s nuts.)
The only work-around I can see is to leave Do Not Disturb mode on all the time, while specifying people on your contact list who are still allowed to ring you when Do Not Disturb is on.