Fire up a wireshark / tcpdump of a transfer and look to see if the TCP window size is limiting the transfer by the laptop. It might not be able to receive as much data.
Look to another service to test the speed. Your test seems web based, have you tried iperf3? You can also play with options in iperf3 (sending udp, tcp, different payload sizes, etc.)
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.
🔥 This is fine 🔥