@bvbw: How did you root the new firmware? TWRP does not work for me....
What do you mean by TWRP does not work ?
Did you use correctly the scatter file from the new ROM ?
New firmware should be rooted the same than previous firmware ... Nothing change !
You need to follow the different steps of various tutorials from internet.
I am on Nougat 7 and i've just flashed the Recovery (TWRP) with SP_Flash_Tool_exe_Windows_v5.1712.00 without problem except unresponsive screen when reboot in TWRP, but easy to solve with a OTG mouse.
Try it and tell me if you have problems.
If needed i'll could share a package with all tools and links useful for a complete working ROOT for BV6000 Nougat (SP Flash tools, Recovery TWRP, SuperSU, Viper4Android 5, BusyBox)