Can't login via gui, but can ssh

Actually you can use sysupgrade on Stock Firmware.
Follow the steps listed on https://openwrt.org/toh/linksys/linksys_wrt32x under Return to Stock Firmware/CLI.

Return to Stock Firmware
Return to OEM

CLI

    Login to OpenWrt via SSH (DropBear / OpenSSH)
        Linux/BSD: SSH client built in
        Windows: Download PuTTY
    Download/Upload firmware image to router
        Download via SSH Terminal:

        cd /tmp && opkg update && opkg install wget && wget <Image-Link>

        – OR –
        Upload via Client Machine Terminal:

        scp -r -i "C:\OpenWrt.ppk" -2 -scp -P 22 "D:\Path\To\Firmware.img" root@192.168.1.1:/tmp

            For PuTTY, change:  scp to  pscp -scp 

            -scp  = force use of SCP protocol
            -r = copy directories recursively
            -i = private key file for user authentication
            -2 = force use of particular SSH protocol version [2]
            -p = connect to specified port

    Flash image
        SSH Terminal:

        cd /tmp && sysupgrade -F -n -v <image-name>

            -F = force write, required to bypass metadata check
            -n = do not save configuration over reflash
            -v = more verbose

I spent quite a lot of time trying to get this to work, to no avail. After some time away, I'm finally getting back to it.

Current state is I have the front face off and the serial cable has arrived, so I need to install OpenWRT on it. I think this is via TFTP.

Problem is I can't find the docs that show the step-by-step of how to do this with a serial cable. Can anyone point me to them?

https://openwrt.org/toh/linksys/linksys_wrt32x under debricking and keep on reading.