select only prefered dns servers on dhcp

Question

I am connecting to a number of wi-fi networks via the network-manager-applet

For debugging purposes, I want to select my dns providers, so that no other dnd provider other than the ones selected is active.

So I edited /etc/dhcp/dhclient.conf, adding the line

supersede domain-name-servers 8.8.8.8, 8.8.4.4;

And, sure enough, all ipv4 dns servers disappeared, except for those two

Running systemd-resolve --status, I see

Link 3 (wlp1s0)
      Current Scopes: DNS
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: no
    DNSSEC supported: no
         DNS Servers: 8.8.8.8
                      8.8.4.4
                      2804:14c:11:672:201:6:2:20
                      2804:14c:10:672:201:6:2:193

However, two things still worry me. The DNS Servers still include some v6 addresses that I have not been able to purge or replace, and systemd-resolve --status has a heading called DNSSEC NTA, with a lot of address-looking entries that I dont know the meaning of

      DNSSEC NTA: 10.in-addr.arpa
                  16.172.in-addr.arpa
                  168.192.in-addr.arpa
                  17.172.in-addr.arpa
                  18.172.in-addr.arpa
                  19.172.in-addr.arpa
                  20.172.in-addr.arpa
                  21.172.in-addr.arpa
                  22.172.in-addr.arpa
                  23.172.in-addr.arpa
                  24.172.in-addr.arpa
                  25.172.in-addr.arpa
                  26.172.in-addr.arpa
                  27.172.in-addr.arpa
                  28.172.in-addr.arpa
                  29.172.in-addr.arpa
                  30.172.in-addr.arpa
                  31.172.in-addr.arpa
                  corp
                  d.f.ip6.arpa
                  home

My question: how do I make sure that the OS (and firefox, if there is any difference there) uses only the dns servers I specify?

OBS: I dont need a method that actually changes configurations. A temporary change that does not get undone by the OS for 10 minutes or so would do just as well.

Also, I am not sure I am looking in the right place. Is my interpretation of the output of systemd-resolve --status correct?