Hello and thanks for getting back
0. updated to latest
apt udpate && apt upgrade
hostnamectl; #
        tested on
          Operating System: Debian GNU/Linux 10 (buster)
                    Kernel: Linux 4.19.0-18-amd64
              Architecture: x86-64
      
1. uninstalled all network managers (now systemd is (probably) the network manager)
2. why does systemd do o much other stuff than startup?
    
3. currently
cat /etc/network/interfaces 
    
configured like this
# === enp2s0:config
        primary network interface with dhcp ===
        allow-hotplug enp2s0
        iface enp2s0 inet dhcp
        dns-nameservers 81.3.27.54
        dns-nameservers 116.202.176.26
        dns-nameservers 1.1.1.1
auto enp2s0:0
        allow-hotplug enp2s0:0
        iface enp2s0:0 inet static
        address 192.168.0.222
        netmask 255.255.255.0
        # gateway 192.168.0.1
    
# 3.1 why is Google's dns coming in first? (how can dns priority
      be configured?)
    
cat /etc/resolv.conf
        
        # Dynamic resolv.conf(5) file for glibc resolver(3) generated by
        resolvconf(8)
        #     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE
        OVERWRITTEN
        nameserver 8.8.8.8
        nameserver 8.8.4.4
        nameserver 81.3.27.54
      
    
Am 16.10.21 um 09:27 schrieb dude:
Hello,
on latest Debian 11 + MATE Desktop (it is software simplicity at it's best :)
why is it not possible to set primary & secondary dns via network-manager-gnome? (only "additional dns")
It is possible. Choose
Method: Automatic (DHCP) addresses only
[Methode: Automatisch (DHCP), nur Adressen]
then specify your DNS servers manually.
also: it used to be /etc/resolv.conf
where nameservers are set
It still is
systemd is doing it’s own thing
/etc/systemd/resolved.conf
Completely unrelated file
well most systems now run systemd, so it is related to the "dns
      config is too complicated and every distro does it's own thing"
      (non standardized?) problem
    
https://www.freedesktop.org/software/systemd/man/resolved.conf.html
best regards