The deed is done. I bought both the routers from the link.
Now time to get the details on how to flash them. Is https://openwrt.org/toh/zyxel/t-56 up to date? I've seen some discussions regarding flashing it that differ slightly, like this one Adding OpenWrt support for Zyxel EX5601-T0 - #305 by bezeria. But this post is a year old so I assume I just go by wiki instructions, right?