They recommend something like this:

# ip link add link eth0 name macvtap0 type macvtap
# ip link set dev macvtap0 promisc on
# ip link set dev macvtap0 up

But then in https://68kmla.org/bb/index.php?threads/tashrouter-an-appletalk-router.46047/page-5#post-518796

Someone says:

I experimented with the recommended macvtap interface but found it to be somewhat poorly documented.
In theory it's supposed to function like a bridge+tap, but I could not get that to work properly.

For whatever reason his code only supports running on a tap/macvtap

router = Router('router', ports=(
  LtoudpPort(seed_network=1, seed_zone_name=b'LToUDP Network'),
  TashTalkPort(serial_port='/dev/ttyAMA0', seed_network=2, seed_zone_name=b'TashTalk Network'),
  MacvtapPort(macvtap_name='macvtap0', seed_network_min=3, seed_network_max=5, seed_zone_names=[b'EtherTalk Network']),
))

  systemd.network = {
    networks = {
      "10-eth-priority" = {
        matchConfig.Name = "enp196s0";
        DHCP = "yes";
        dhcpV4Config.RouteMetric = 1023;
        dhcpV6Config.RouteMetric = 1023;
      };
      "30-enp196s0" = {
        matchConfig.Name ="enp196s0";
        linkConfig = {
          Unmanaged = "yes";
        };
        linkConfig.RequiredForOnline = "enslaved";
        networkConfig = {
          Bridge = "br0";
        };
      };
      "40-tap0" = {
        matchConfig.Name ="tap0";
        bridgeConfig = {   };
        linkConfig.RequiredForOnline = "enslaved";
        networkConfig = {
          Bridge = "br0";
        };
      };
      "40-br0" = {
        matchConfig.Name = "br0";
        bridgeConfig = {};
        linkConfig = {
          RequiredForOnline = "carrier";
        };
      };
    };
    netdevs = {
      "20-tap0" = {
       enable = true;
        netdevConfig = {
          Kind = "tap";
          Name = "tap0";
        };
      };
      "20-br0" = {
        enable = true;
        netdevConfig = {
          Kind = "bridge";
          Name = "br0";
        };
      };
    };
  };

  systemd.network = {
    networks = {
      "10-eth-priority" = {
        matchConfig.Name = "enp196s0";
        DHCP = "yes";
        dhcpV4Config.RouteMetric = 1023;
        dhcpV6Config.RouteMetric = 1023;
      };
      "30-enp196s0" = {
        matchConfig.Name ="enp196s0";
        linkConfig = {
          Unmanaged = "yes";
        };
        linkConfig.RequiredForOnline = "enslaved";
        networkConfig = {
          Bridge = "br0";
        };
      };
      "40-tap0" = {
        matchConfig.Name ="tap0";
        bridgeConfig = {   };
        linkConfig.RequiredForOnline = "enslaved";
        networkConfig = {
          Bridge = "br0";
        };
      };
      "40-br0" = {
        matchConfig.Name = "br0";
        bridgeConfig = {};
        linkConfig = {
          RequiredForOnline = "carrier";
        };
      };
    };
    netdevs = {
      "20-tap0" = {
       enable = true;
        netdevConfig = {
          Kind = "tap";
          Name = "tap0";
        };
      };
      "20-br0" = {
        enable = true;
        netdevConfig = {
          Kind = "bridge";
          Name = "br0";
        };
      };
    };
  };

❯ networkctl 
IDX LINK       TYPE     OPERATIONAL SETUP      
  1 lo         loopback carrier     unmanaged
  2 enp196s0   ether    routable    configured 
  4 wlan0      wlan     routable    configuring
  6 tailscale0 none     routable    unmanaged
  7 tap0       ether    no-carrier  configured 
  8 br0        bridge   no-carrier  configuring

7: tap0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc fq_codel master br0 state DOWN group default qlen 1000
    link/ether 66:f4:2d:a9:1b:fb brd ff:ff:ff:ff:ff:ff
8: br0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default qlen 1000
    link/ether 16:3d:f3:58:00:14 brd ff:ff:ff:ff:ff:ff
    inet6 fe80::143d:f3ff:fe58:14/64 scope link proto kernel_ll 
       valid_lft forever preferred_lft forever

2: enp196s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 06:e0:4c:68:00:9b brd ff:ff:ff:ff:ff:ff
    altname enx06e04c68009b
    inet 192.168.5.156/24 metric 1023 brd 192.168.5.255 scope global dynamic enp196s0
       valid_lft 86262sec preferred_lft 86262sec

bridge link
7: tap0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 master br0 state forwarding priority 32 cost 2 

...
      "30-enp196s0" = {
        matchConfig.Name ="enp196s0";
        linkConfig = {
          Unmanaged = "yes";
        };
        linkConfig.RequiredForOnline = "enslaved";
        networkConfig = {
          Bridge = "br0";
        };
      };
...