And actually, I ran into trouble getting this to work even with SD cards and enabling gadget mode, I think something isn't quite configured correctly. I made the changes to dhcpc.conf and /etc/systemd/network and when I power on p1, any ideas?
==> /var/log/kern.log <==
Jan 20 05:17:37 pi4 kernel: [ 6786.682528] usb 1-1.3.4: new full-speed USB device number 8 using xhci_hcd
Jan 20 05:17:38 pi4 kernel: [ 6787.642548] usb 1-1.3.4: new high-speed USB device number 9 using xhci_hcd
Jan 20 05:17:38 pi4 kernel: [ 6787.773629] usb 1-1.3.4: New USB device found, idVendor=0525, idProduct=a4a2, bcdDevice= 5.04
Jan 20 05:17:38 pi4 kernel: [ 6787.773639] usb 1-1.3.4: New USB device strings: Mfr=1, Product=2, SerialNumber=0
Jan 20 05:17:38 pi4 kernel: [ 6787.773647] usb 1-1.3.4: Product: RNDIS/Ethernet Gadget
Jan 20 05:17:38 pi4 kernel: [ 6787.773655] usb 1-1.3.4: Manufacturer: Linux 5.4.83+ with 20980000.usb
Jan 20 05:17:38 pi4 kernel: [ 6787.815547] cdc_ether 1-1.3.4:1.0 usb0: register 'cdc_ether' at usb-0000:01:00.0-1.3.4, CDC Ethernet Device, 4a:ea:6f:9d:e3:b7
Jan 20 05:17:38 pi4 kernel: [ 6787.815707] usbcore: registered new interface driver cdc_ether
Jan 20 05:17:38 pi4 kernel: [ 6787.815849] usbcore: registered new interface driver cdc_subset
==> /var/log/daemon.log <==
Jan 20 05:17:38 pi4 systemd-udevd[2475]: Using default interface naming scheme 'v240'.
Jan 20 05:17:38 pi4 dhcpcd[539]: br0: fe80::968f:cfff:fe7c:b550 is unreachable
==> /var/log/kern.log <==
Jan 20 05:17:38 pi4 kernel: [ 6787.904582] br0: port 2(usb0) entered blocking state
Jan 20 05:17:38 pi4 kernel: [ 6787.904590] br0: port 2(usb0) entered disabled state
Jan 20 05:17:38 pi4 kernel: [ 6787.904750] device usb0 entered promiscuous mode
Jan 20 05:17:38 pi4 kernel: [ 6787.906380] br0: port 2(usb0) entered blocking state
Jan 20 05:17:38 pi4 kernel: [ 6787.906389] br0: port 2(usb0) entered forwarding state
Jan 20 05:17:38 pi4 kernel: [ 6787.939340] br0: port 2(usb0) entered disabled state
==> /var/log/daemon.log <==
Jan 20 05:17:38 pi4 systemd-networkd[392]: usb0: Gained carrier
Jan 20 05:17:38 pi4 systemd-networkd[392]: usb0: Configured
Jan 20 05:17:38 pi4 systemd-networkd[392]: usb0: Lost carrier
Jan 20 05:17:39 pi4 dhcpcd[539]: br0: soliciting an IPv6 router
Jan 20 05:17:39 pi4 dhcpcd[539]: br0: Router Advertisement from fe80::968f:cfff:fe7c:b550
Jan 20 05:17:39 pi4 dhcpcd[539]: br0: fe80::968f:cfff:fe7c:b550 is reachable again
Jan 20 05:17:40 pi4 avahi-daemon[307]: Joining mDNS multicast group on interface usb0.IPv6 with address fe80::48ea:6fff:fe9d:e3b7.
Jan 20 05:17:40 pi4 avahi-daemon[307]: New relevant interface usb0.IPv6 for mDNS.
Jan 20 05:17:40 pi4 systemd-networkd[392]: usb0: Gained IPv6LL
Jan 20 05:17:40 pi4 avahi-daemon[307]: Registering new address record for fe80::48ea:6fff:fe9d:e3b7 on usb0.*.
Jan 20 05:17:49 pi4 dhcpcd[539]: br0: fe80::968f:cfff:fe7c:b550 is reachable again
Jan 20 05:17:49 pi4 dhcpcd[539]: br0: fe80::968f:cfff:fe7c:b550 is reachable again