Thinking about it, there is a way out - flashing escalade's community builds from the ZyXEL OEM firmware:
http://luci.subsignal.org/~trondah/lede/ipq806x/r8532/
Which is recent enough to contain a full-featured factory image including luci - and can be used to downgrade to an official 18.06.1 sysupgrade image using the webinterface. Making this an all graphical (webinterface based) flashing method.