Well, the description above works, mostly. You need to retain a symlink to "files" from "files-4.19" when creating the "change from files to files-4.19" commit. Some part of the build system uses the wrong location:
ln -s files-4.19 ./target/linux/lantiq/files
Do this after the "rm -rv ./target/linux/lantiq/files" and before the "git add ...".
Furthermore, the sysupgrade image has a different idea of supported devices than the one from Quallenauge. However, that is just a name change. You can override it by passing the "-F" option to sysupgrade. On top of that, the resulting image comes with significantly fewer bells and whistles than other images posted around here. That may be a good thing. If you want luci, you need to install it. One rather obnoxious thing, though, is the wifi configuration. It does build the following packages if you successfully point it to the extra feeds before, as described after the build instructions:
./bin/targets/lantiq/xrx200/packages/ralink_bin_1.0.0-1_mips_24kc.ipk
./bin/targets/lantiq/xrx200/packages/kmod-ralink_inic_4.19.108+1.0.0-1_mips_24kc.ipk
./bin/packages/mips_24kc/base/wireless-tools_29-6_mips_24kc.ipk
You need to scp those onto the easybox after the sysupgrade and "opkg install" them. Then you have to run "/etc/init.d/rt3883 boot" after each reboot. I will find a way to automate this ...
Another problem is that the darn tft display gets stuck and never switches itself off. Turning it face down does the trick for now. However, I am one step further. Nice.