Glinet AR300M upgrade to vanilla openwrt

I have a Glinet 300M-ext with NAND flash.

I'd like to upgrade to plain vanilla openwrt, but I'm utterly confused by the steps provided (this and the glinet forum, but mainly trying to this guide: GL-AR300M and GL-AR750S NAND Support (ath79))

so far i've upgraded the glinets own version to latest (3.104 -2020-05-23 12:31:55)
uboot upgrade does not provide option for flashing nor or nand, i.e. one upgrade option only.

I've tried nand/nor img/bin files via uboot/sysupgrade methods but without success.
Could someone provide the steps for my specific device?