Not trying to go down a rabbit hole, nor invade your teen’s privacy, but have you done any kind of packet inspection on what’s going out/in? Teens can surprise you with the kind of stuff they’re up to sometimes.

I’m not sure why your resolver started acting up but what you’re describing doesn’t sound like normal cause/effect. Four people on a residential connection, even if you throw in a ton of electronic devices and iot/crap that calls home constantly shouldn’t cause any kind of ISP engagement.

Not like it really matters, for 99.9% of people having a forwarder is easy and just fine and there isn’t good reason to troubleshoot it if there’s a working solution. I’m pretty privacy conscious and I don’t even think having my own forwarder is worth the hassle, I am just choosy about my upstream.