I've followed https://gist.github.com/varhub/7b9555cdd1e5ad785ffde2300fcfd0bd and have doublechecked that the build.prop is indeed modified i.e. system was mounted in read-write mode.
Unfortunately after reboots, the /data/property/persist.sys.usb.config resets itself to just mtp (instead of mtp,adb).
Any ideas what I am missing? I have to do this via adb since I am working on a headless phone with a broken screen (touch/display not working at all).