Initial steps done for grouper-nakasi
Had to manually download TWRP and SU packages - there seems to be problem with NRTK linked files.
It uses rather outdated versions of TWRP and SU. But I stuck with this older versions (SR3-SuperSU-v2.79-SR3-20170114223742 and twrp-3.0.2-0-grouper) for now.
You said busybox is optional, anyway I run into trouble with the install:
I choose the su/xbin destination, but busybox won't install, says "Systemless root is not configured"