Good idea! Thanks for the help.
Still a bit dazzled that there are no 'real' primary and secondary DNS servers and that they simply co-exist.
Would it also be possible to use more than 2 DNS servers? And how does PiHole decide what query to send where? Random?
instant edit: yes, I checked their online documentation (kb.netgear.com) as well as the user forums but so far haven't been able to find anything about it. In the future I might flash DD-WRT on it, but I don't like flashing away my warranty since I just bought it.