Since the behaviors you are experiencing don't appear to fit the logic that we would expect, my recommendation would be to start from scratch.
Start by generating a custom image that is just the standard 24.10.2 with the addition of the USB-ethernet driver, then write that to a card.
Insert the card, plug in the USB ethernet driver, power up the Pi, and plug your computer into the built-in ethernet port. Your computer should get an address in the 192.168.1.0/24 network.
Next, add eth1 to br-lan (if the system has auto-created new interfaces (wan/wan6) using eth1, just delete those).
Reboot the Pi, keeping your computer connected to the built-in ethernet port. Once the Pi has finished booting, you should, once again, have an IP address from Pi and you should be able to reach it (ping, ssh, LuCI web interface). Once that is confirmed, unplug the cable from the built-in ethernet port and connect it to the USB-ethernet adapter. Does your computer get an IP from the Pi at this point, and can it reach it normally?