Why is Pi-Hole enabled in NethServer?
AFAIK NethServer implements a very basic DNS that just forwards requests.
(the lack of “real” DNS server has been discussed before in this forum)
In any case Technitium is a full DNS server, so it implements most if not all kinds of records, TXT, SRV etc. (not just A) and can act as authoritative DNS. Not sure if Pi-Hole can do that.
Also Technitium works with DNS-over-TLS and DNS-over-HTTPS.