🔥 This is fine 🔥

Well, your Guest Wi-Fi can either have access to the pihole server and have its benefits, or point them to a public DNS. You can’t block access to the dns server for your guest network and hope it works - because it’s blocked. Personally I’d give Guest Wi-Fi a public DNS, as you won’t have much fun when an app of your friend’s stop working and you have to fiddle and work it out.

If you point the guest network’s dns to the gateway, that gateway needs to know what to do with those requests. Either it has a resolver locally or it mist forward it elsewhere.

I was surprised to find a few years ago that phones were allowed in classes. It makes absolutely no sense to me. What is the positive side to letting them in?

I’m not sure if you’re joking or not.